A sensitive and specific microenzyme immunoassay (EIA) procedure for porcine brain natriuretic peptide (BNP)-like immunoreactivity has been developed. Enzyme-labeled antigen was prepared by conjugation of synthetic BNP with beta-D-galactosidase using N-(epsilon-maleimidocaproyloxy)succinimide method. Using a second antibody-coated immunoplate, the minimum amount of BNP-like immunoreactivity (BNP-LI) detectable by this assay system was 1.6 fmol/well. When porcine BNP-LI in porcine plasma was assayed by the present method levels between 1 and 8 pmol/l were detected. Gel filtration of porcine plasma extracts on Sephadex G-25 revealed the presence of two immunoreactive peaks; one eluted at a position identical with that of BNP-26 and the other eluted earlier, close the position of BNP-32.