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Fifteen to twenty percent of patients with juvenile rheumatoid arthritis (JRA) have positive latex fixation tests (LFT), whereas approximately 46% have previously been demonstrated to have hidden rheumatoid factors (RF), i.e., 19s IgM RF which can be detected by the LFT after acid separation of the IgM-containing fraction from serum. In this study, hidden RF were found in 59% of patients with seronegative JRA by use of a complementdependent hemolytic assay. The median titer of JRA patients was l :42, and in healthy and disease controls it was 1 :7. The difference was significant at P < 0.001. When data from patients with active disease were analyzed separately, the median titer for polyarticular JRA was l :97 and for pauciarticular JRA, 1 :91. The differences due to active disease were significant at P < 0.001 and P < 0.005, respectively.The results demonstrate that the hemolytic assay is more sensitive than the LFT in determining the presence of hidden RF, and activity of disease correlates well with high hemolytic R F titers.