Background. The current goal of human immunodeficiency virus type 1 (HIV-1) therapy is to maximally suppress viral replication. Securing this goal requires new drugs and treatment classes. The chemokine receptor CCR5 provides an entry portal for HIV-1, and PRO 140 is a humanized monoclonal antibody that binds to CCR5 and potently inhibits CCR5-tropic (R5) HIV-1 in vitro. Methods. A randomized, double-blind, placebo-controlled, dose-escalating study was conducted in 39 individuals with HIV-1 RNA levels ≥5000 copies/mL, CD4 + cell counts ≥250 cells/μL, no antiretroviral therapy for 3 months, and only R5 HIV-1 detectable. Cohorts were randomized 3:10 to receive placebo or doses of PRO 140 of 0.5, 2, or 5 mg/kg. Subjects were monitored for 58 days for safety, antiviral effects, and serum concentrations of PRO 140. Results. PRO 140 was generally well tolerated and demonstrated potent, rapid, prolonged, and dose-dependent antiviral activity. Mean reductions in HIV-1 RNA level of 0.58 log10, 1.20 log10 (P = .0002) and 1.83 log10 (P < .0001) were observed for the 0.5-, 2-, and 5-mg/kg dose groups, respectively. Reductions in mean viral load of ≥10-fold were observed within 4 days and persisted for 2-3 weeks after treatment. Conclusions. This trial established clear proof of concept for PRO 140 as a potent antiretroviral agent with extended activity after a single dose. Trial registration. ISRCTN Register: ISRCTN45537485. © 2008 by the Infectious Diseases Society of America. All rights reserved.