Using an antiserum raised in rabbits to a synthetic human brain natriuretic peptide (BNP) N-terminal fragment [ProBNP(1-13)], a single large molecular weight (MW) N-terminal form of ProBNP has been identified in human plasma. Sep-pak extracts of plasma, drawn from patients with congestive heart failure and subjected to size exclusion and reverse phase high pressure liquid chromatography (HPLC) coupled to radioimmunoassay (RIA) revealed a single large immunoreactive (ir-) peak which was not detected in assays of BNP-32 [ProBNP(77-108)]. This material has a MW of 8600--similar to that expected for the complete N-terminal portion of ProBNP(1-76). Previously reported high MW BNP forms, cross reacting with BNP-32 antisera, were not detected using the N-terminal antisera, indicating that this material is unlikely to be intact ProBNP(1-108). In patients with congestive heart failure plasma ir-levels of N-terminal ProBNP were greatly raised compared to normal subjects and were up to ninefold higher than ir-BNP-32 values.