Purified IgM rheumatoid factors (RF; 3 monoclonal and 2 polyclonal) were shown to inhibit, in a dose-dependent manner, 2 complement-mediated functions, i.e., the immune complex solubilization capacity and the inhibition of immune precipitation. Inhibition of immune complex solubilization capacity occurred only if RF was added at the same time as, but not after, addition of the complement source. Experimental evidence suggests that the effects of RFs were not related to their anticomplementary activity, but rather required the attachment of RF to the Fc region of the IgG molecule. Although no clinical data are available so far, it might be plausible that these newly described properties of RF have biologic relevance.