A direct capture enzyme-linked immunosorbent assay (ELISA) was developed to measure elevated polymorphonuclear granulocyte (PMN) antigens using horseradish peroxidase (EC 126.96.36.199) conjugated rabbit polyclonal anti-PMN antisera and a monoclonal antibody specific for PMN cells. Optical densities obtained in the ELISA were used to predict the cell counts of milk samples. Predicted counts were not significantly different from actual somatic cell counts (SCC). In a total of 156 bovine milk samples the correlation coefficient between somatic cell counting, taking > 500000 cells/ml as being indicative of mastitis, and the assay was 0·94, yielding an assay sensitivity of 95·2% and a specificity of 97·3%. In further trials the ELISA could detect elevated PMN antigens in milk with SCC as low as 100000 cells/ml. The results indicate that the monoclonal antibody based direct ELISA has excellent potential in the detection and determination of bovine mastitis.