One-hundred twenty-five serum samples from 82 patients with juvenile rheumatoid arthritis (JRA) were studied for the presence of hidden rheumatoid factor (RF) in an effort to find a better serologic marker to define JRA. Hidden 19S IgM RF was detected by means of a hemolytic assay utilizing the IgM-containing fraction of serum. The IgM fraction was obtained after acid separation of serum on a Sephadex G-200 column. Hidden 19S IgM RF was present in 68% of patients with seronegative JRA with a mean titer of 1:63. The mean titer for the polyarticular JRA group was 1:83, for the pauciarticular JRA group, it was 1:32, and for the systemic type-onset JRA patients, it was 1:32. When disease was active, the mean titer for all JRA patients was 1:108, for the active polyarticular JRA group it was 1:119, for the active pauciarticular JRA, it was 1:97, and for the active systemic JRA patients, it was 1:64. All values were significant at the P less than or equal to 0.001 when compared to disease and normal controls. The hemolytic assay for RF on the IgM-containing fraction of serum thus enhances the serologic capabilities of defining JRA.