Antibody responses to poliovirus, diphtheria, pertussis, or tetanus vaccine were compared in five groups of infants. The 62 infants had been brought up on breast milk or on one of four types of artificial feed in the first five months of life. The types of artificial feed varied in quality and quantity of protein; they were high or low protein cow's milk, an adapted formula (with a casein/albumin ratio of 40/60), and a formula based on soy flour. After the age of 5 months, all infants were put on the same diet. The general pattern of antibody responses as determined by antibody levels when the infants were 5 and 8 months old was that those fed on breast milk or high-protein cow's milk had adequate and sustained antibody responses; those fed on the adapted formula had a high but temporary response; and those fed on low-protein cow's milk or the soy-based formula had poor responses.