A solid-phase radioimmunoassay capable of detecting nanogram quantities of human IgM rheumatoid factor using a monoclonal anti-mu-chain antibody is described. Human IgG did not interfere with the detection of IgM RF by this method. The small nonspecific binding of nonRF IgM to the human IgG coated tubes utilized in the assay must be corrected for by assaying samples in parallel bovine serum albumin coated control tubes only in cases of deviation of IgM from normal range. 69 coded and randomly arranged sera from patients with rheumatoid arthritis (RA), nonrheumatic joint diseases and healthy adult control subjects were investigated by this method, agglutination techniques as well as RIPEGA. A good correlation between solid-phase radioimmunoassay and agglutination techniques was found. Patients with seropositive RA had significantly higher concentrations of IgM RF than seronegative RA patients or control subjects (mean +/- 1 SD = 133,3 +/- 187,2 micrograms/ml versus 4,7 +/- 6,5 micrograms/ml and 2,2 +/- 4,0 micrograms/ml; resp.).