An animal model of neonatal protein deprivation was developed to examine the effects of maternal malnutrition on growth and development and on the host defense system of the suckling offspring. Adult rats were fed either a protein-deficient (3% casein) or normal (25% casein) diet beginning one day after parturition. Offspring of the protein-deprived animals showed biochemical signs of nutritional imbalance such as changes in serum acid hydrolase levels as early as the second day of life; growth retardation and hypoproteinemia developed by day 4. When malnourished and control sucklings were infected at 12 days of age with Staphylococcus aureus, it was noted that protein deprivation did not influence neutrophil mobilization. However, malnourished animals responded to infection with larger perturbations in neutrophil counts than did the controls, were unable to control the infection, and ultimately showed neutrophil depletion. These studies suggest that protein deprivation affects the quantity and quality of milk and that the offspring of a protein-deficient animal are not only growth retarded but are also compromised in their ability to deal with infection.