Serum levels of IgG subclasses were measured in 18 children with acute lymphoblastic leukemia (ALL) receiving maintenance chemotherapy and in 36 age-matched controls in order to attempt to analyse the effects of chemotherapy. The IgG subclasses were measured by enzyme linked immunosorbent assay. Serum IgG1, IgG2 and IgG4 levels in the patients were significantly (p less than 0.01, p less than 0.005, p less than 0.005) lower than in the controls, but serum IgG3 levels in patients were as high as in controls. Suppression on IgG2 and IgG4 were more profound than IgG1. In six children, the levels of the IgG subclasses were measured at diagnosis, during maintenance chemotherapy and one year after cessation of chemotherapy. The levels of the four IgG subclasses at diagnosis and after cessation of chemotherapy were as high as those in control children except for the IgG4 levels in the post-chemotherapy group. IgG2 and IgG4 may be more susceptible to suppression by chemotherapy than IgG1 and IgG3 may not be suppressed by chemotherapy.