We isolated human brain natriuretic peptide (human BNP) from the human atrium. Sequence analysis has revealed that it is a 32-amino-acid peptide with the sequence S-P-K-M-V-Q-G-S-G-C-F-G-R-K-M-D-R-I-S-S-S-S-G-L-G-C-K-V-L-R-R-H, which is identical to the C-terminal sequence (77-108) of the human BNP precursor deduced from the cDNA sequence. The sequence of human BNP (77-108) is preceded by Pro75-Arg76 in the human BNP precursor, which is the same processing signal as Pro97-Arg98 of the precursor of atrial natriuretic peptide (ANP). The processing of the BNP precursor occurs in the cardiocyte, although that of the ANP precursor in the cardiocyte is unclear at present.