An isotype-specific ELISA for detection of rheumatoid factors (RFs) is described. The assay utilizes rabbit anti-albumin IgG bound to albumin as solid phase reactant (test wells). RF-activity was expressed as the difference in reaction between test wells and wells containing F(ab')2-fragments of the rabbit IgG bound to albumin. The activity correlated well with the conventional Waaler agglutination test on sera that showed a positive Waaler titre, but several sera that were negative by the Waaler test were positive for RF of IgG isotype by the ELISA-test. The method was applied on specimens from psoriatic patients, and RFs of IgG- and IgA isotype were demonstrated in serum immune complexes from all the psoriatic patients examined. IgG- and IgA-RF was also demonstrated in extracts from psoriatic scale, and RFs in scale were demonstrated independently of free rheumatoid factor activity in serum.