Male and female guinea pigs receiving, respectively, 100 and 280 mg/day Vitamin C throughout the duration of immunization showed enhanced humoral antibody production to bovine serum albumin (BSA) and, in castrated females, to penicilloyl-coupled guinea pig gamma-globulin. A significant degree of protection was afforded against fatal anaphylactic shock in passively sensitized males. Under appropriate conditions of sensitization with rabbit anti-HGG and challenge with human gamma-globulin, 8 of 20 unsupplemented animals died of shock, whereas in the group receiving 280 mg Na ascorbate/day for 4 days preceding passive transfer and again 60' before challenge, only 2 of 18 died. The rate of dose-dependent mortality observed when groups of passively sensitized animals were challenged with increasing doses of antigen was reduced in animals supplemented as above. Actively immunized guinea pigs were not protected by 5 daily doses of 280 mg Na ascorbate given prior to challenge. There were no significant differences in the total hemolytic activity of the serum nor in the C3 and C4 components of complement in immunized animals. There was no change in the concentration of Cl esterase in non-immunized controls, but immunization with BSA was followed by a rapid decline in Cl concentration, the decrease being greater in the ascorbate-treated group than in the unsupplemented controls, possibly reflecting a higher level of circulating immune complexes in the former case.