HIV-1 B cell epitopes from gp41, the T cell epitope of p34pol, and a cluster of B and T epitopes from p17gag were selected. The epitopes were presented as synthetic peptides and as either N- or C-terminal insertions into beta-galactosidase. Hybrids were efficiently expressed in E. coli and easily purified when epitopes were inserted at the beta-galactosidase C terminus. Sera from HIV-1-infected individuals reacted in peptide- and hybrid protein-based enzyme-linked immunosorbent assays (ELISAs) mostly with the immunodominant site of gp41. The second site of gp41 and also sites from p17 and p34 appeared to be immunorecessive. A few of the HIV-1-positive sera exhibited several immunorecessive reactivities. HIV-1-positive sera from the former Soviet Union and Cuba had reactivities similar to those of American, African, and west European sera. Some sera could not be evaluated as specifically HIV-1 seropositive because of their broad reactivities with a multitude of peptides and proteins, unrelated to HIV-1. Extensive tests were performed to define unspecific reactivities by absorption, blocking, and sandwich ELISAs. The application of the hybrid protein assay substantially improved the specificity of the ELISA tests. Thus, hybrid protein-based ELISAs appeared to be more suitable than peptide-based ELISAs, especially for the evaluation of immunorecessive reactivities.