The immunotropic activities of human lactoferrin were studied with respect to phenotypic and functional changes in murine splenic B cells. Phenotypic changes were induced by human lactoferrin in splenic B-cell fractions separated by buoyant density. B cells from 7-8-day-old BALB/c mice isolated from a 50/60% Percoll gradient, gained characteristic features of more mature B cells manifested by an increase of surface IgD and complement receptor expression. Incubation of the analogous B-cell fraction from adult mice with human lactoferrin resulted in minor changes in relation to IgM and IgD expression. Besides induction of phenotypic changes on immature B cells, human lactoferrin enabled B cells from normal newborn and adult immunodeficient CBA/N mice to present antigen to an antigen-specific T-helper type 2 (Th2) cell line. We conclude that human lactoferrin acts as a maturation factor for B cells with regard to their phenotype and function.