The ability of an αCD4-pokeweed antiviral protein (PAP) immunoconjugate to inhibit replication of human immunodeficiency virus type 1 (HIV-1) was evaluated in vitro with 22 clinical HIV-1 strains obtained from four seropositive asymptomatic individuals, three patients with AIDS-related complex, and four patients with AIDS. Fifteen isolates were from zidovudine- untreated individuals, whereas seven isolates were obtained after 24 to 104 weeks of therapy with zidovudine, alone or alternating with zalcitabine. Mean zidovudine 50% inhibitory concentrations (IC50s) were 126 nM (range, 1 to 607 nM) for isolates from zidovudine-untreated individuals and 2,498 nM (range, 14 to 6,497 nM) for strains from patients treated with antiretroviral agents. Mean αCD4-PAP IC50s were 48 x 10-3 nM (range, 0.02 x 10-3 to 212 x 10-3 nM) for isolates from zidovudine-untreated individuals, and 16 x 10-3 nM (range, 2 x 10-3 to 28 x 10-3 nM) for isolates from treated patients. Overall, higher concentrations of αCD4-PAP were necessary to inhibit HIV-1 strains from untreated individuals at more advanced stages of disease. Seventeen isolates were susceptible to zidovudine (mean IC50, 117 nM), and five were resistant to zidovudine (mean IC50, 3,724 nM). Mean αCD4-PAP IC50s were 43 x 10-3 nM for zidovudine-susceptible isolates and 19 x 10-3 nM for isolates resistant to zidovudine. All HIV-1 strains had IC50s greater than 0.5 nM for unconjugated PAP, the αCD19-PAP immunoconjugate, and monoclonal antibody αCD4. At concentrations as high as 5,000 nM, αCD4-PAP did not inhibit colony formation by normal bone marrow progenitor cells (BFU-E, CFU-GM, and CFU-GEMM) or myeloid cell lines (KG-1 and HL-60) and did not decrease cell viabilities of T-cell (Jurkat) or B- cell (FL-112 and Raji) precursor lines. Overall, αCD4-PAP demonstrated more potent anti-HIV-1 activity than zidovudine and inhibited replication of zidovudine-susceptible and zidovudine-resistant viruses at concentrations that were not toxic to lymphohematopoietic cell populations.