Large numbers of maternal lymphocytes are present in breast milk. We asked whether exposure of an infant to maternal lymphocytes during the process of breast feeding would have an effect on the subsequent reactivity of a patient to a maternal-donor related renal transplant. We studied the posttransplant course of 55 patients who had received a primary maternal-donor transplant. Twenty-seven recipients had been breastfed during infancy and 28 recipients had not been breast-fed. A history of breast feeding was associated with a more favorable posttransplant course as measured by the percentage of patients who had no rejection episodes during the first posttransplant year (P [less than or greater than]. 006). The one-year graft function rate for breast-fed recipients was 82%; this was statistically significantly better than the 57% measured for non-breast-fed recipients (P [less than or greater than]. 05). Statistical significance of differences between groups was not attained when results were evaluated over a five-year interval. A difference between breastfed and non-breast-fed recipients was not apparent when we evaluated a somewhat smaller group of patients who had received a paternal donor transplant. From these observations we conclude that the process of breast feeding during infancy may result in a measurable immunologic benefit to the recipient of a subsequent maternal-donor related renal transplant.