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 complement-dependent hemolytic assay. The median titer of JRA patients was 1:42, and in healthy and disease controls it was 1:7. The difference was significant at P less than 0.001. When data from patients with active disease were analyzed separately, the median titer for polyarticular JRA was 1:97 and for pauciarticular JRA, 1:91. The differences due to active disease were significant at P less than 0.001 and P less than 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 RF titers.