The effects of vitamin A and vitamin E supplementation on the IgG response to tetanus toroid after primary immunization were evaluated in a prospective, randomized controlled clinical trial involving 89 healthy infants with normal serum vitamin A and E levels at 2 months of age. Before the first dose of DPT vaccine, the infants were randomly enrolled into four different study groups [Group I (n = 24): 30,000 IU vitamin A for 3 days just after each three doses of primary vaccination, Group II (n = 21): 150 mg oral vitamin E for only 1 day after the injections for primary immunization, Group III (n = 21): vitamins A and E together in the same order, Group IV (n = 23) no vitamin after DPT vaccines]. Serum tetanus antitoxin (IgG) titres were measured three times; initially at 2 months of age before the first dose of DPT, secondly at 5 months of age 1 month after primary immunization and thirdly at 16-18 months of age before the booster dose of DPT. Before the first dose of the DPT vaccine, 1 month after the third DPT injection and at 16-18 months before the booster dose of DPT, there was no significant difference in serum tetanus antitoxin levels between these four groups. A significant increase was observed in all the groups when serum tetanus antitoxin levels before (2 months) and after (5 months) primary immunization were compared. In addition, serum antibody levels against tetanus significantly decreased in the four groups before booster vaccination. Before the beginning of primary immunization, 15 infants (16.8%) had serum tetanus antitoxins (IgG) below protective level. After three doses of DPT, all the infants had protective antitoxin levels. At 16-18 months of age before booster dose, four infants (10%) also had serum tetanus antitoxins (IgG) below the protective level. No side-effects were observed except bulging fontanelle in two infants in Group I. (C) 2000 Elsevier Science Ltd. All rights reserved.