An enzyme-linked immunoassay for human C4b-binding protein (concentration range 25-400 ng/ml) was developed using two monoclonal antibodies; the intra- and interassay coefficients of variation were less than 7.2%. In 50 normal subjects, 20 patients with liver cirrhosis and 34 full-term newborns, the plasma levels of C4b-binding protein were very similar to those measured by Laurell electroimmunoassay; in 24 patients with elevated erythrocyte sedimentation rates, levels measured by enzyme-linked immunoassay were higher then those measured by the Laurell method (270 +/- 70% vs. 223 +/- 67%). In these patients crossed immunoelectrophoresis showed a pattern different from that of normal individuals, which may explain the lower values found with the Laurell method.