We investigated serum IgA, IgG, IgM and IgG subclass concentrations, complement activity, lymphocyte subpopulations, lymphocyte responses to mitogens and natural killer cell cytotoxicity in 15 children who had had osteomyelitis and septic arthritis and in a group of control subjects. IgG subclass concentrations below the fifth percentile for age occurred in 4 patients. We found isolated deficiencies of IgG1 and IgG3 and combined deficiencies of IgG2/IgG3/IgG4 and IgG2/IgG4. Concentrations of IgA and of IgG tended to be below the mean for age. The percentage of Leu-11+ cells was reduced in patients. Other immunological parameters studied were normal. These findings suggest that although most patients who have had osteomyelitis and septic arthritis do not have low immunoglobulin concentrations, impaired antibody production may be a predisposing factor in at least a few such children.