Serum IgG subclass levels were determined using a competitive indirect immunoenzymatic assay with monoclonal antibodies in 16 patients with ataxia-telangiectasia. Eight children had IgA deficiency, two had IgG and IgA deficiency and six patients showed no immunoglobulin class abnormality. However, IgG4 and IgG2 levels were undetectable or low in almost every patient. An IgG3 deficiency was associated with the IgG2-IgG4 defect in three patients with undetectable IgA. IgG1 was very low in one patient with a total IgG deficiency. There was no clear correlation between subclass levels and the occurrence of infections.