Symptoms after routine primary immunisation of 6004 infants with diphtheria/tetanus/pertussis (DTP) vaccine and 4024 infants with diphtheria/tetanus (DT) vaccine have been compared. After each dose, crying, screaming, and feverishness were more frequent with adsorbed DTP than adsorbed DT, but the difference was small. Attacks of high-pitched screaming, episodes of pallor or cyanosis with limpness, convulsions, and local reactions occurred with similar frequency after both vaccines. There was a considerable increase in local reactions after the 3rd dose with both vaccines. Over 1000 doses of plain DTP (with no aluminium hydroxide adjuvant) were given during the study. Post-vaccination symptoms were more common after the plain than after the 2 adsorbed preparations.