The relative frequencies of IgM antiIgG autoantibody (rheumatoid factor) producing cells induced by the polyclonal B cell activator Epstein-Barr virus were measured in peripheral blood lymphocyte cultures of normal children and patients with juvenile rheumatoid arthritis. The frequencies of rheumatoid factor precursor B cells in normal children were lower than adults, but higher than neonates. The frequency increased with the age of the donor. In seronegative children with the systemic-onset or pauciarticular-onset types of juvenile rheumatoid arthritis, the number of IgM antiIgG inducible B cells was not significantly different (P greater than 0.05) from age-matched controls. Patients with seropositive juvenile rheumatoid arthritis or seropositive adult rheumatoid arthritis had significantly higher IgM antiIgG precursor cell frequencies than age-matched normal subjects (P less than 0.01 and P less than 0.02, respectively). In contrast, the patients with seronegative polyarticular-onset juvenile rheumatoid arthritis had an average precursor frequency significantly lower than normal age-matched controls (P less than 0.05), analogous to results previously noted in adult seronegative rheumatoid arthritis. Thus, both children and adults with seronegative polyarticular rheumatoid arthritis had a deficiency in B cells that produce IgM antiIgG and that are induced by Epstein-Barr virus. This distinguished them from seropositive juvenile rheumatoid arthritis and rheumatoid arthritis patients, normal subjects, and patients with the pauciarticular-onset and systemic-onset types of seronegative juvenile rheumatoid arthritis.