['Despite the comparatively conserved nature of the HIV-1 gag gene, countless quasispecies of the p17 gene coexist in HIV-1-infected patients. It is not known if the minor genetic differences in quasispecies will affect immune recognition.', 'To characterize the antigenicity and immunogenicity of three different members of HIV-1 p17 quasispecies.', 'Three members of HIV-1 p17 gene quasispecies, one from patient A (clone 9; qsA9) and two from patient E (clones 5 and 8; qsE5 and qsE8), were expressed and purified from Escherichia coli. The antigenicity of the p17 proteins was analysed using sera from HIV-1-infected individuals, and the immunogenicity was evaluated using sera and lymphocytes from primed mice of three different haplotypes.', 'The antigenicity of the qsE5 and qsE8 p17 recombinant proteins were distinct when tested for reactivity with human p17 antibodies. The qsE5 and qsE8 p17 were equally immunogenic in H-2d mice, but not in H-2b and H-2k mice. In H-2b mice the qsE8 protein induced higher levels of anti-p17 IgG2a, IgG2b and IgG3 than the qsE5 protein. Corroborating the IgG subclass pattern, H-2b-restricted qsE5-specific T cells produced higher in vitro levels of interferon-gamma, but not of interleukin (IL)-4, IL-5 and IL-6, than qsE8-specific T cells, suggesting a more pronounced T-helper (TH)1-like response.'] Do coexisting members of HIV-1 p17 gene quasispecies represent proteins with distinct antigenicity and immunogenicity?