Superoxide dismutase (SOD) activity has been estimated in serum and leukocytes isolated from adults and children with acute lymphoblastic leukemia (ALL), non-Hodgkin lymphoma (NHL), Hodgkin's disease (HD) and neuroblastoma. Superoxide dismutase levels in leukocytes isolated from adult patients with HD, ALL and NHL were significantly higher than the corresponding control values. Serum SOD activity of adult patients with ALL and NHL was significantly decreased compared to the normal value, while its activity was not significantly changed in patients with HD. Marked deficiencies of leukocyte SOD levels in children with ALL, NHL, HD and neuroblastoma were found. There were no apparent differences in serum SOD activity in children suffering from ALL, NHL, HD and neuroblastoma. It is concluded that the assay of leukocyte SOD can be used to differentiate between HD and NHL in both adults and children.