Antigen--antibody complexes were detected in patients' sera by inhibition of the agglutinating activity of rheumatoid sera toward IgG coated particles (latex). Precision, sensitivity (1--10 microgram/ml equivalents of heat-aggregated IgG), and reproducibility (maximum coefficient of variation of 11%) were obtained by measuring agglutination with an instrument counting the residual free particles. Automation allowed testing of 20 to 40 samples per hour. The inhibitory activity of spontańeously agglutinating sera was determined after inactivating endogenous rheumatoid factor by reduction with dithiothreitol. Non-aggregated IgG did not significantly interfere. The agglutinating activity of 6 rheumatoid sera was tested after incubation with the various immunoglobulin classes and subclasses polymerized by coupling to agarose, and all were found to be readily absorbed by IgG1, but poorly by IgG3, IgA1 and IgM. Reactivity with IgG2, IgG4 and IgA2 clearly differed for different rheumatoid sera. Among 70 sera from blood donors, 7 had abnormally high inhibitory activity. Six of these had also an abnormal protein profile, suggesting existence of latent disease. High inhibitory activity in 15 sera out of 18 from patients with systemic lupus erythematosus, and in 23 sera out of 46 from patients with breast cancer suggested that the rheumatoid factor inhibition test has a discriminatory capacity comparable with that of more sophisticated techniques requiring radioisotopes and/or cellular material.