The aim of this study was to determine whether breast milk had a stimulating effect on the development of immune responses in infants. Breast- and bottle- fed infants were studied from birth to 9 months of age. Several significant differences in immune responses were found between the groups. These differences were related to age and fell into two main time periods. In the early neonatal period, lymphocytes from breast-fed infants showed significantly greater spontaneous proliferation and proliferative responses to the T cell mitogen PHA to vitro than cells from bottle-fed infants. This may be due to a stimulatory effect to vivo of the growth factors and lymphokines in human milk acting on T cells and/or their precursors. Serum immunoglobulin levels did not however, reflect this increased lymphocyte responsiveness and although salivary IgM and IgA levels were significantly increased in the breast-fed infants at 6 days of age, this may have been due to residual milk immunoglobulins. In contrast, by 3 to 9 months of age, the cells from the bottle-fed group showed significantly greater to vitro proliferation of all classes of lymphocytes than the cells from the breast- fed infants. Salivary IgM and IgA levels and serum IgM antibodies to commensal gut organisms were also significantly higher in bottle-fed infants at this time. This indicated a higher rate of to vivo stimulation of the immune system in bottle-fed infants. It is suggested that this is due to an increased exposure of bottle-fed 'infants to antigenic material at mucosal surfaces and a greater uptake of these local antigens into the systemic circulation. Breast-feeding may therefore have contrasting effects on the development of immune responses, a stimulating effect by growth factors, particularly in the early immature period, and a suppressive effect resulting indirectly from exclusion of antigens (including those of a potentially harmful nature).