Hidden 19S IgM rheumatoid factors (RF)-i.e., RF detected in the IgM-containing fraction after separation of the serum at an acid pH-have been found in 68% of patients with seronegative juvenile rheumatoid arthritis (JRA). Inhibition studies utilizing a hemolytic assay for RF were performed to determine the specificity of hidden 19S IgM RF. Sera from 14 children with JRA were separated by gel filtration at pH 4.05. Two were seropositive for RF and 12 were seronegative; the latter had high titer hidden 19S IgM RF. The IgM-containing fractions were preincubated with monomeric human IgG, rabbit IgG, or bovine IgG, and the complement-dependent hemolytic assay ws performed. The RF in the IgM fraction from the 2 seropositive patients were inhibited most strongly by rabbit IgG, whereas hidden RF in the IgM fraction of 9 seronegative patients were inhibited markedly by human IgG (homologous IgG equal to autologous IgG), poorly by rabbit IgG, and not at all by bovine IgG. Further inhibition studies with the hidden 19S IgM RF demonstrated inhibition by the human IgG1 subclass in all patients and only minimal inhibition by the IgG3 subclass in 3 patients. Inhibition with IgG1 Fc fragments produced by papain and thermolysin digestion demonstrated inhibition by only those fragments that contained the G1m (a) antigenic area which is found in the C gamma 3 homology area of the IgG1 molecule. These data indicate that hidden 19S IgM RF possibly circulate as immune complexes bound to the IgG1 molecule and the binding chiefly occurs in th G1m (a) homology area.