The present invention relates to an immunoassay for detection of BNP, proBNP and fragments thereof. Essentially the assay comprises: a) contacting the antigen with a first antibody specific to a fragment corresponding to amino acids 11-22 of BNP, or to a part of this peptide comprising at least three amino acids of said sequence, to obtain a first order immune complex. b) contacting the first order immune complex obtained at step (a) with a second antibody recognizing said first order immune complex, to obtain a second order immune complex, wherein said antibody is unable to recognize free BNP, proBNP or free first antibody; c) Detecting the second order immune complex.