In this report, solid phase radioimmunoassays that specifically measure IgG rheumatoid factor (RF) and IgM RF in absolute concentrations are described. Polyclonal RF preparations were utilized as standards, and as little as 40 ng/ml of IgG RF and I ng/ml of IgM RF were detected. The IgG RF concentration of 26 seropositive rheumatoid sera was 439 +/ 755 micrograms/ml (mean +/- 1 SD), a level 107 times that of normal controls (P less than 0.001). In contrast, levels for 49 children with juvenile rheumatoid arthritis (JRA) were 6.1 +/- 3.7 micrograms/ml for those with a polyarticular onset (JRA-Po), and 27.3 +/- 113 micrograms/ml for the pauciarticular group (JRA-Pa), and 12.6 +/- 20.7 micrograms/ml for the group with a systemic onset (JRA-S). None of these values differed significantly from the value of 6.0 +/- 3.9 micrograms/ml measured in a juvenile control group. The IgM RF level in adult rheumatoid arthritis (RA) of 175 +/- 221 micrograms/ml was also significantly elevated compared to controls (P less than 0.001). In JRA, however, mean levels of 1.4 +/- 2.0 micrograms/ml (JRA-Po), 2.8 +/- 8.3 micrograms/ml (JRA-Pa), and 1.1 +/- 0.7 micrograms /ml (JRA-S) were not elevated significantly above the value of 1.2 +/-1.2 micrograms/ml measured in the juvenile control group. Hidden IgM RF was not found in 9 JRA sera tested. These marked differences in RF levels provide another indication that adult RA and JRA are distinct diseases.