The vaccine response to poliovirus, diphtheria and tetanus toxoids in relation to protein intake was studied in infants, either breast-fed or given low (1.1 g/100 ml) or conventional (1.5 g/100 ml) protein formula. Serum, saliva and faeces antibodies were measured by the enzyme-linked immunosorbent assay. Neutralizing poliovirus antibodies were determined. The serum, saliva and faeces antibody responses in the two formula-fed groups of infants did not differ significantly, but for the low protein formula group which had significantly higher serum neutralizing titres to poliovirus after the second vaccine dose than the conventional formula group. However, the breast-fed group had significantly higher antibody levels than the two formula-fed groups together: serum IgG to diphtheria toxoid (p less than 0.01) and serum neutralization of poliovirus (p less than 0.001) at 21-40 months of age, saliva secretory IgA to tetanus (p less than 0.01), diphtheria toxoid (p less than 0.01) and poliovirus (p less than 0.05), as well as faecal IgM to tetanus toxoid (p less than 0.05) and poliovirus (p less than 0.01 and p less than 0.05) at 3 and 4 months of age. Breast-fed infants thus showed better serum and secretory responses to peroral and parenteral vaccines than the formula-fed, whether with a conventional or low protein content.