To identify the potential precursor(s) of HIV-1 nucleocapside protein (NC; p7) and matrix protein (MA; pl7). Peptides FNCGKEGHTARNC, deduced from a conserved region of NC, and GHSNOVSQNY from c-terminal MA were conjugated to keyhole limpet hemocyanin and inoculated into New Zealand white rabbits to produce anti-peptide antibodies. The specificity of the antibodies was determined by enzyme linked immunosorbent assay (ELISA) and Western blot. ELISA showed that anti-MA and anti-NC peptide antibodies reacted with MA and NC peptides respectively. They recognized recombinant DNA derived HIV-1 proteins which contain sequences of these peptides, but did not react with other proteins such as bovine serum albumin. Treatment of commercial HIV-1 Western blot strips (Biotech/DuPont and Bio-Rad) with the antibodies showed that anti-MA and anti-NC peptide antibodies recognized different proteins at pl7. In Western blot assays, the binding of anti-MA was blocked by an excess amount of MA but not NC peptide, while the binding of anti-NC peptide antibodies was selectively inhibited by NC peptide. These results suggest that there is a potential pl7 precursor of NC which comigrates with MA in HIV-1 Western blots.