IgG-containing antigen-antibody complexes were detected in the sera of all of 7 patients with the systemic form of juvenile chronic arthritis, usually at high levels. Only 9 of 25 patients with the polyarticular form had such complexes, usually at low levels. Three patients had low levels of serum IgA which were probably drug induced; the one patient with low C2 and the 3 with low yeast opsonisation were probably not more than to be expected in a random population.