A 134-mer peptide corresponding to the N-terminal sequence of p24 (residues 146-279 of the gag gene product of the LAV strain) was chemically synthesized using highly optimised protocols on an ABI 430A synthesizer. The crude peptide was obtained by treating the peptide-resin with HF, then purified by a combination of size exclusion and RP-HPLC. One hundred milligram of 90% pure 134-mer can be obtained within a month. Both mice and rabbit polyclonal antisera raised against a commercial preparation of recombinant p24, and a pooled sera from HIV-1 infected individuals reacted strongly with the 134-mer peptide in ELISA. Both mice and rabbits immunized with the free peptide emulsified in Freund's complete adjuvant generated strong anti-peptide and anti-p24 antibody responses as judged by immunoblots and ELISAs. Immunodominant epitopes were mapped to residues 201-227 (LKETINEEAAEWDRVHPVHAGPIAPG). These B-cell epitopes had previously been identified by mouse monoclonal antibodies raised against HIV-1 virus or gag gene products. Furthermore, murine T-cell lines generated against the 134-mer peptide were found to respond to two short peptides, P24B (residues 195-215) and P24D1 (residues 268-279). These two T-cell epitopes were previously reported as human helper T-cell and CTL epitopes, respectively. These results clearly indicate that the synthetic 134-mer peptide could elicit both T- and B-cell responses to HIV-1 similar to those obtained with the natural viral gag protein, and could be useful for the development of a synthetic HIV vaccine.