A sensitive and rapid enzyme-linked immunosorbent assay for simultaneous quantitation of free and IgA-complexed human protein HC was developed with monoclonal murine antibodies. The total amount of protein HC (free plus IgA-complexed) was measured by a competitive procedure while the protein HC-IgA complex was quantitated by a sandwich enzyme immunoassay. The amount of free protein HC was then obtained as the difference between the 2 measured values. The sensitivity of the procedure was 70 μg/l for the total amount of protein HC and 80 μg/l for the protein HC-IgA complex. At ordinary human serum level of free protein HC and protein HC-IgA complexes the coefficient of variation for the procedure was about 5%. One worker could determine the concentrations of free and IgA-complexed protein HC in up to 400 serum samples in a working day.