The purpose of this study is:

1. To assess the antiviral activity of PRO 140
2. To assess the safety and tolerability of PRO 140
3. To generate additional PK, PD and safety data of PRO 140

Inclusion Criteria:

1. Males & females, age ≥ 18 years (or minimum adult age as determined by local regulatory authorities)
2. Screening plasma HIV-1 RNA ≥ 5,000 copies/mL
3. CD4+ lymphocyte cell count ≥ 300 cells/mm3 and no documented count < or = 250 cells/mm3
4. Has not taken any antiretroviral therapy (ART) w/in 12 wks of Early Screening Visit
5. Exclusive CCR5-tropic virus as determined by Trofile™ Assay at Early Screening Visit
6. Clinically normal or "not clinically significant (NCS)" resting electrocardiogram
7. Women of reproductive potential must have a negative serum pregnancy test at Late Screening Visit & a negative urine pregnancy test w/in 72 hrs prior to first dose of study medication, & be non-lactating. Male & female subjects must agree not to participate in a conception process from Early Screening Visit through Day 59.

Exclusion Criteria:

1. CXCR4 tropic virus or dual/mixed tropic (R5X4) virus determined by the Trofile™ Assay.
2. Females who are pregnant, lactating or breastfeeding, or who plan to become pregnant during the study.
3. History of active hepatitis within the previous 24 wks
4. Prior use of any entry, attachment, CCR5 co-receptor or fusion inhibitor, including PRO 140, experimental or approved.
5. Any immunization or vaccination (including influenza vaccine) within 30 days prior to administration of study drug.