A prospective study of immunogenicity and adverse effects of 1553 doses of diphtheria and tetanus toxoids and whole cell pertussis vaccine (DTP) was performed in 538 children observed longitudinally from 2 months to 20 months of age. Subjects were randomized to the standard four-dose immunization schedule or to a three-dose schedule (with a saline injection substituted for DTP at 6 months of age). The three-dose schedule could not be recommended on the basis of serologic data. Compliance for completing a clinical observation form in the 48 hours following injections was greater than 99%. Fever, local reactions, or adverse behavioral effects were described in association with 96% of DTP doses and 36% of placebo injections. Contraindications to DTP immunization developed in 3% of study children. No convulsion, hypotonic hyporesponsive episode, encephalopathy, or temperature greater than 40.5 degrees C occurred. Behavioral and local inflammatory effects occurred maximally in the first 6 hours following vaccine but fever peaked later. There was no interrelationship between occurrence of local reaction and fever. Data suggest that age has more effect on the type and rate of adverse clinical events than does vaccine dose number. Existing antibody levels to vaccine components, lot of vaccine, breast-feeding, or gestational age did not affect rate or type of clinical reactions. Neither occurrence of reactions nor the use of acetaminophen affected antibody response to vaccine.