Data of 46 patients with juvenile chronic arthritis were evaluated. All of them were under the age of 16 at the time of study. Patients with juvenile ankylosing spondylitis, psoriatic arthritis, and arthritis associated with inflammatory bowel disease were excluded from this series. Twenty-six patients had pauciarticular, 9 had systemic, 8 had polyarticular, and 3 patients had seropositive 'adult type' JCA. In all patient groups IgG, IgM, IgA, and C3 levels were significantly higher than in the control group. None of the controls but 13 of 46 patients had high IgE levels. IgA and IgG values were significantly higher in the seropositive subgroup than in the pauciarticular subgroup. There was no other positive correlation between levels of immunoglobulins, C3, and clinical features of disease. Platelet counts were significantly higher in patients with active disease. In the control group 4 out of 10 children had 'hidden' rheumatoid factor. All these children with positive findings were young.