We have investigated the humoral immune response to Sheep Red Blood Cells (S.R.B.C.--a T-dependent cellular antigen) in two groups of random-bred guinea pigs. A control group of animals was maintained on "minimal" ascorbate intake, while the test group animals received supplementary vitamin C in their drinking water. Serum haemolytic antibody levels were monitored at various intervals subsequent to immunization with S.R.B.C. The primary antibody response of the ascorbate-supplemented animals differed statistically significantly from that of the controls in two respects. The peak antibody titre was both higher and occurred earlier in the test animals. No significant difference in the antibody response to a secondary injection of antigen was manifest. Our observations suggest that vitamin C stimulates the elaboration of Ig M-type antibodies in particular.