RATIONALE: Interleukin-12 may kill tumor cells by stopping blood flow to the tumor and by stimulating a person's white blood cells to kill breast cancer cells.

PURPOSE: Randomized phase II trial to study the effectiveness of interleukin-12 in treating patients who have metastatic or recurrent breast cancer.

OBJECTIVES: I. Determine the activity of interleukin-12 as defined by the percentage of patients who have not progressed after 6 months of therapy. II. Compare percentage of patients who have not progressed after six months with or without treatment regimen. III. Determine time to progression and overall survival in this patient population after this treatment.

OUTLINE: This is a randomized study. Patients are stratified according to disease free interval from primary diagnosis to first metastases (less than 3 years vs 3 years and longer), estrogen receptor status (positive vs negative), and disease status (complete response, partial response, detectable disease, or stable disease). Patients are randomized to one of two treatment arms. Arm I: Patients begin therapy no sooner than 3 weeks and no later than 6 weeks since last chemotherapy dose. Patients receive interleukin-12 subcutaneously twice a week. Treatment continues in the absence of disease progression or unacceptable toxicity. Patients are followed at least every 3 months for 1 year. If no progression after 1 year, may be followed as needed for new signs or symptoms and survival for 5 years. Arm II: Patients are observed for 6 months. If disease progresses during first 6 months, patients may receive interleukin-12 as in arm I. Patients without disease progression within first 6 months may also then receive interleukin-12 as in arm I. Treatment continues in the absence of disease progression or unacceptable toxicity. Patients are followed for toxicity only until interleukin-12 is discontinued.

DISEASE CHARACTERISTICS: Histologically confirmed metastatic or recurrent breast cancer (not confined to breast) One prior induction chemotherapy regimen for recurrent or metastatic disease (4 weeks of treatment for 4-6 courses) resulting in stable disease, partial response, or complete response Pleural or peritoneal effusion palliated by induction chemotherapy allowed No uncontrolled brain metastases Previously treated brain metastases allowed if: No evidence of progression for at least 3 months following radiotherapy and/or surgical treatment AND At least 30 days since dexamethasone or other corticosteroids AND Other metastatic site exists No bone marrow or brain as only sites of metastases No meningeal disease Hormone receptor status Estrogen receptor status known

PATIENT CHARACTERISTICS: Age: 18 and over Performance status: ECOG 0-1 Menopausal status: Not specified Life expectancy: Not specified Hematopoietic: WBC at least 3,000/mm3 Absolute neutrophil count at least 1,500/mm3 Hemoglobin at least 9 g/dL (transfusion or epoetin alfa allowed) Platelet count at least 100,000/mm3 Hepatic: SGOT/SGPT no greater than 1.5 times upper limit of normal (ULN) Albumin at least 3.0 g/dL Bilirubin no greater than 1.5 times ULN Renal: Creatinine no greater than 1.5 times ULN Creatinine clearance at least 60 mL/min Other: No other active malignancy except nonmelanoma skin cancer Not pregnant or nursing Fertile patients must use effective contraception No inflammatory bowel disease or active gastric ulcer No prior autoimmune disease or immunodeficiency syndrome

PRIOR CONCURRENT THERAPY: Biologic therapy: See Chemotherapy No concurrent interleukin-11 Chemotherapy: See Disease Characteristics No concurrent chemotherapy Prior chemotherapy with bone marrow progenitor support (bone marrow transplant and/or peripheral blood stem cell support) allowed in adjuvant setting only (not for metastases) Endocrine therapy: Prior hormone therapy for breast cancer allowed No concurrent hormones allowed except for non-cancer or cancer treatment related conditions (e.g. insulin for diabetes) No concurrent dexamethasone or other steroidal antiemetics Radiotherapy: Prior radiotherapy allowed No concurrent radiotherapy Surgery: Not specified Other: Prior bisphosphonate therapy allowed if started at least 3 months before study No initiation of bisphosphonate therapy during study