Abstract Using tanned human red cells coated optimally with isolated plasma proteins, specific agglutination in an AutoAnalyzer was quantitatively inhibited by solutions of the coating protein. IgG, IgM, IgA, IgD, IgE, transferrin, fibrinogen and ceruloplasmin were assayed in this manner. Lowest detectable concentrations varied from 0.2 µg IgE/ml and 0.4 µg IgG/ml to 9 µg IgD/ml. The amount of 125I-labeled IgG bound by tanned cells was a linear function of the amount applied when both variables were treated as logarithms, and about 2000 bound molecules per cell were optimal for agglutination. IgG appeared to bind at its Fab region, but less than 5% of the lowest applied concentrations was bound. Assays of cerebrospinal fluid for IgG compared favorably with quantitative precipitin data, but some myeloma proteins were not assayed reliably in reference to normal IgG.