The purpose of this study is to assess the safety and tolerability of AMG 139 following multiple intravenous (IV) or subcutaneous (SC) dose administration in healthy subjects and in subjects with mild to severe Crohn's disease (CD).

Inclusion Criteria:

Part A - Healthy Subjects:

• Healthy male or female of non-reproductive potential subjects between 18 to 45 years-of-age
• Body mass index (BMI) between 18 and 32 kg/m2
• Normal or clinically acceptable physical examination, clinical laboratory values, and ECG
• Additional inclusion criteria apply

Part B - Crohn's Subjects:

• Male or female subjects with CD between 18 to 55 years-of-age
• Body mass index (BMI) between 18 and 32 kg/m2
• Normal or clinically acceptable physical examination (PE), clinical laboratory values, and ECG; clinically acceptable PE includes evidence of mild to severe CD
• Diagnosis of ileal or ileo-colonic CD for a minimum of 6 months
• Mild to severe CD defined by a Crohn's Disease Activity Index (CDAI) score equal to or greater than 180 and equal to or less than 450
• Elevated fecal calprotectin and CRP
• Additional inclusion criteria apply

Exclusion Criteria:

Parts A - Healthy Subjects:

• History or evidence of a clinically significant disorder (including psychiatric), condition or disease that would pose a risk to subject safety or interfere with the study evaluation, procedures or completion;
• Recent or on-going infection(s)
• Underlying condition(s) that predisposes the subject to infections
• History of malignancy of any type
• Additional exclusion criteria apply

Part B only - Crohn's Subjects:

• History or evidence of a clinically significant disorder (including psychiatric), condition or disease that would pose a risk to subject safety or interfere with the study evaluation, procedures or completion;
• Recent or on-going infection(s)
• Underlying condition(s) that predisposes the subject to infections
• History of malignancy of any type
• Additional exclusion criteria apply