AbstractA variety of phagocytic cell and lymphocyte assays were employed to evaluate the immune status of four patients with methylmalonicaciduria. One patient had a depressed absolute granulocyte count and two patients had depressed neutrophil and monocyte chemotactic responses. All subjects had normal neutrophil phagocytic and bactericidal activities. One patient had a decreased T‐cell number; blastogenic responses to phytohaemagglutinin and pokeweed mitogen were normal in all subjects. B lymphocyte measurements were variably abnormal; two children had decreased B‐cell numbers; two had marginally decreased IgG levels; a third had an undetectable rubella titre; and two had elevated serum IgE concentrations.In vitro exposure of normal cells to methylmalonic acid concentrations up to 50mg/100 ml did not affect chemotactic or lymphoproliferative responses.In conclusion, although B‐cell function may be affected, no consistent abnormality of lymphocyte or phagocytic cell functions could be attributed to the metabolic disorder.