Type III pneumococcal polysaccharide (SSSIII) did not cross-react with the nucleic acid antigen polyinosinic · polycytidylic acid when tested in NZB or CBA mice. Neither antigen requires thymic helper cells for the induction of an antibody response. The antibody response to each antigen, given alone or together, was increased by simultaneous administration of antithymocyte serum. Studies of the immune response to SSSIII were conducted in NZB mice since it was felt that this autoimmune strain might represent a natural model for the loss of suppressor T cells. The number of plaque-forming cells to SSSIII following immunization increased with age in female NZB mice. No such age-associated increase was observed in non-autoimmune BALB/c mice, suggesting that NZB mice might have a loss of suppressor function with age. This was supported by the reduction in the high PFC response to SSSIII of 10-month-old NZB mice by transfer of thymocytes from 4-week-old NZB mice.