The effect on BCG immunisation of feeding either formula or breast milk was assessed in Canadian Cree infants who were vaccinated either at birth or after 1 month of age. The response to BCG was measured in terms of lymphocyte blastogenesis stimulated by purified protein derivative of Mycobacterium tuberculosis. Breast-feeding significantly enhanced cell-mediated immune response to BCG vaccine given at birth, but had no significant effect if vaccine was given after 1 month. These findings were not related to maternal history of tuberculosis or BCG vaccination, and the feeding method did not influence lymphocyte stimulation by candida or streptococcal antigens.