Human milk lactotransferrin at a concentration ranging from 1 to 10 micrograms/ml stimulated up to 5 times the humoral immune response to sheep red blood cells, expressed as the number of plaque-forming cells, when injected into mice 3 h before immunization. Further, lactotransferrin-treated thymocytes given intravenously into mice, enhanced the immune response to sheep red blood cells to the same extent as IL-1. In vitro, studies showed that CD4- CD8- thymocytes incubated with lactotransferrin and added to the splenocyte cultures, increased the immune response to sheep red blood cells. Flow cytometry analysis studies indicated that, after an overnight incubation with human lactotransferrin, CD4- CD8- thymocytes acquired the CD4 antigen characteristic for the helper cell phenotype. Taken together, these results suggest that lactotransferrin stimulates the immune response by a process which involves the promotion of T cell differentiation.