Nine monoclonal anti C-reactive protein (CRP) antibodies were tested as peroxidase-conjugates in a CRP binding to a solid-phase phosphorylethanolamine (PE) assay. One monoclonal antibody was selected due to its high yield of conjugate, titre and stability. The use of monoclonal antibodies increased assay sensitivity, precision and allowed assay simplification by the simultaneous rather than sequential incubation of CRP and conjugate. Comparison with nephelometry and CRP binding to PE assay using polyclonals gave correlations greater than or equal to 0.9. By using monoclonal conjugates to assay human sera, healthy adult CRP levels were found to be lower than by using polyclonal conjugates. Samples with very low CRP contents might now be assayed with higher precision.