Various assays were used to examine chemotaxis, opsonization, phagocytosis, and killing of Salmonella organisms by colostral and breast-milk components. Vigorous responses of colostral a;d milk cells against this organism and nonspecific opsonizing capacity of the aqueous phase of colostrum and milk were demonstrated. An assay with acridine orange was used to directly visualize the phagocytic and killing processes; colostral and milk cells were demonstrated to be more active against Salmonella than blood neutrophils. In a retrospective survey, a diagnosis of Salmonella infection was confirmed in 253 infants less then 1 year of age. Only 12 infants had ever been breast-fed, and only one near the time he had the disease. During the same period of time, 27% of matched control infants were breast-fed.