We measured serum concentrations of immunoglobulin classes and IgG subclasses in 53 patients who had completed treatment for Hodgkin's disease and in 10 healthy adults. We wished to determine the relation of these classes and subclasses to the subjects' antibody responses to bacterial polysaccharide and viral protein antigens. Mean levels of the IgG-2 subclass were significantly lower in patients treated with both radiation and chemotherapy than in controls (P less than 0.05). The level of IgG-2 before immunization correlated directly with the mean antibody response both to 11 pneumococcal antigens (r = 0.71, P less than 0.001) and to the Haemophilus influenzae Type b antigen (r = 0.40, P less than 0.01). The correlation between IgG-2 concentration and pneumococcal antibody response was also significant in the 10 healthy adults ( r = 0.70, P less than 0.05). The levels of no immunoglobulin class or subclass correlated significantly with antibody responses to influenza A/Victoria/75 and A/New Jersey/76 hemagglutinins, both of which are protein antigens. The serum concentration of IgG-2 appears to provide a marker for predicting the ability to produce antibodies to polysaccharide, but not viral protein, antigens.