We have prepared a monoclonal antibody to Lipoprotein(a) [Lp(a)] and have used it to develop an ELISA test for assaying Lp(a) in serum. The monoclonal antibody employed in the assay system reacts uniformly with S1 ,S2,S3 and B phenotypes of isoforms, and no cross-reaction with plasminogen at a concentration of 100 mg/dL was observed. Results of the monoclonal ELISA assay were similar to those obtained with a polyclonal antibody ELISA method and demonstrated a correlation coefficient, r=0.99 with the equation for the regression line:Y(proposed)= l .06 X (polyclonal antibody reference ELISA test) = 0.36(N = 51). Inter-and intra-assay precision(CVs) of the monoclonal ELSA assay were between 2.2-3.6% at a mean Lp(a) concentration range of 19.1-68.2 mg/dL,(N = 12).Assay results of various standards were compared by both monoclonal and polyclonal antibody ELISAtests. We observed some discrepancies between expected concentrations and the polyclonal antibody ELISA assay results, which is thought to be more uniformly reactive to the various Lp(a) phenotypes.The monoclonal antibody employed in our proposed method reacts uniformly with Lp(a) phenotypes, and the assay exhibits excellent sensitivity, specificity, and accuracy and is well suited for clinical use. o 1995 Wiley-Liss, Inc.