Routine laboratory tests (Rose-Waaler test, latex fixation test) for the detection of rheumatoid factor (RF) measure IgM RF by its ability to agglutinate sheep red blood cells or latex particles coated with IgG. Recently, solid-phase RIA and ELISA have been developed to detect RF of all major immunoglobulin classes in a sensitive and reproducible manner. In the literature the pathogenetic, diagnostic and prognostic significance of IgA, IgG and IgM RF is discussed. Using a micro-ELISA with human IgG-Fc-fragments as antigens, we have detected IgA RF and/or IgG RF not only in sera from seropositive rheumatoid arthritis (RA) (n = 48), but also in 13 of 22 sera from seronegative RA, which was statistically significant compared with a group of 50 normal controls. There was a good correlation between IgM RF in ELISA and Rose-Waaler titres for agglutinating RF (r = 0.70). No RF were found in patients with psoriatic arthritis (n = 16).