Transfer of serum proteins separated by thin layer agarose electrophoresis onto nitrocellulose sheets precoated with purified human polyclonal IgG followed by revelation with enzyme-coupled anti-mu or anti-alpha antisera resulted in the specific detection of rheumatoid factors (RF) belonging to the IgM or IgA classes. Mono- or polyclonality of such RF can be evaluated from the patterns of the blots (sharp bands). In addition, their light chain type can be determined using affinity filters coated with a gamma heavy chain disease protein or with IgG Fc fragments. This simple and rapid procedure allows an easy characterization of monoclonal RF, even if they are present in minute amount amongst polyclonal RF as in certain sera from rheumatoid arthritis patients.