Forty-one normal full-term infants were fed from birth, during the first 4.5 months of life, different diets based on two formulas similar in rough composition but basically different, the one being constituted only of cow milk protein and the other of soy protein. From each of the two formulas two different dilutions were prepared so that all diets supplied about 100 kcal/kg/day, but, respectively, 2.0 and 4.0 g/kg/day of cow milk protein and 2.0 and 5.0 g/kg/day of soy protein. After 4.5 months, growth in weight, length, and head circumference was normal and very similar in all infants. Gammaglobulin, immunoglobulin, transferrin, and some complement fractions (particularly C'3, C'1 INA, C'3 PA) were lower in infants receiving soy protein diets than in those receiving cow milk protein diets, and, within each type of diet, in those infants receiving lower amounts of protein. In particular, values of infants receiving 5.0 g/kg/day of soy protein were roughly comparable to those of infants receiving 2.0 g/kg/day of cow milk protein. B lymphocyte markers and reactivity did not show significant differences among the various groups. T lymphocyte markers and reactivity showed an impairment in soy protein diets (and in particular in 2.0 g/kg/day protein diets) with respect to cow milk diets. It was also observed that morbidity (mainly infections of upper respiratory tract) was higher in those infants who took soybean protein and in those who took lesser amount of protein.