Ch7 (RGSDIAG), a synthetic heptapeptide derived from a conserved region of HIV p24 (aa 232-238), was previously shown to suppress antigen-induced responses in cultures of normal human peripheral blood lymphocytes (PBL). We show in this paper that Ch7 is the shortest peptide retaining full inhibitory capacity. Further, the peptide inhibited efficiently and in a dose-dependent manner the induction of a specific antibody response to the antigens SRC (sheep red cells) and Candida albicans but did not exert any effect on the induction of immunoglobulin-secreting cells in PWM-stimulated cultures. Finally, Ch7 inhibited anti-CD3-induced lymphoproliferation but did not affect anti-CD2 activation. These results suggest that a conserved epitope of HIV p24 may be able to prevent the induction of antigen-specific antibody responses by interfering with lymphocyte activation via the T3-Ti complex, resulting in the abrogation of immune functions that are defective in HIV-infected individuals.