Two chimpanzees were immunized against HIV-1 : C-339, using whole inactivated HIV-1 followed by purified recombinant gp160 and a KLH-V3 peptide conjugate; and, C-499, using purified recombinant gp160 and p18gag followed by a mixture of uncoupled V3 peptides. The antigens were emulsified prior to use with one volume Syntex adjuvant SAF-1 containing 1 mg/ml Threonyl-MDP. The animals were challenged twice one year apart by the intravenous route, the first time with cell-free virus, using 40 chimpanzee infectious units (100 TCID50) of the titrated HTLV-IIIB virus stock from the National Cancer Institute, Frederick, MD; and the second time with cell-associated virus, using 6 x 10(5) viable peripheral blood mononuclear cells (PBMC) from a naive chimpanzee which had been injected 3 months earlier with cell-free virus and had become virus-positive within a few weeks. From end-point titration of C-087 PBMC on indicator human PBMC, using a reverse transcriptase assay, this represented the equivalent of at least 15 infected cells. A 3rd chimpanzee, C-435, a näive animal, was also injected with 6 x 10(5) PBMC from C-087 to serve as a positive control. The PMBC of the animals were co-cultivated with fresh human PBMC and assayed for reverse transcriptase on a regular basis. In parallel, ELISA and Western blot analyses were carried out. Virus was detected in the PBMC from C-435 beginning at 4 weeks after challenge. This was followed by seroconversion of the animal to the Env and Gag antigens. By contrast, no virus could be detected in the PBMC from chimpanzees C-499 and C-339 during 7 and 12 months, respectively. Lymph node biopsies and bone marrow aspirates from these animals remained virus-negative upon co-cultivation with human PBMC. PBMC, bone marrow aspirates and lymph node biopsies also scored HIV-negative by polymerase chain reaction. Finally, no anamnestic antibody response of the animals could be detected by ELISA, and no modification of their western blot profiles that could have signed HIV-infection were observed during the follow-up period. C-499 accidentally developed an infectious endocarditis with congestive liver and kidney failure and had to be euthanized at 7 months post-challenge. Specimens from its brain, kidneys, liver, mesenteric nodes, pancreas, salivary glands, and spleen were processed for co-cultivation with human PBMC. No evidence for the presence of virus could be detected by reverse transcriptase assays in any of these co-cultures.(ABSTRACT TRUNCATED AT 400 WORDS)