A simple enzyme-linked immunosorbent assay (ELISA) inhibition test was devised to determine the separate specificities for rabbit IgG, Fc, and Fab fragments of IgM, IgG, and IgA antiglobulins in sera obtained from rheumatoid and normal individuals. Results of this test showed that most of the anti-rabbit IgG activity present in the three immunoglobulin (Ig) isotype preparations from a rheumatoid serum was specific for the Fc portion of whole IgG. Some anti-Fab activity was detectable in all three Ig isotypes examined, but this had much less avidity and/or specificity than the anti-Fc activity. In contrast, normal antiglobulins of M and G classes were mostly specific for the Fab region of rabbit Ig, although a small but measurable amount of Fc-specific antiglobulin was present and was of high relative avidity. The low normal serum IgA anti-IgG activity detected was essentially nonspecific. We conclude that normal antiglobulins differ from "rheumatoid factors" in their specificity and that this may relate to different roles in health and disease.