Concentrations of immunoglobulins G, M, and A were measured by double-antibody radioimmunoassay in morning milk samples collected during the first month postpartum from 35 mothers delivered of preterm infants and 14 mothers delivered of term infants. Mean concentrations of IgG (1.8, to 2.8 mg/gm protein) and IgM (2.8 to 11.7 mg/gm protein) were similar in milk from both groups of mothers. In contrast, IgA was present in significantly higher concentrations throughout the first month postpartum in milk from mothers delivered of preterm infants than in milk from those giving birth at term (P less than 0.01). To determine the effect of milk flow on IgA concentration, IgA was also measured in complete 24-hour milk collections; milk from mothers with preterm deliveries again contained significantly higher concentrations of IgA than milk from mothers with term deliveries (P less than 0.01). This higher IgA concentration was not secondary to method of milk expression. The concentration of IgA was found, however, to vary inversely with milk volume (P less than 0.01). Although mean values of milk volumes for the groups were not statistically different, the overall lower volumes of milk produced by mothers giving birth preterm resulted in comparable total IgA production per 24 hours. There were no differences in serum IgA concentrations of preterm infants fed their own mother's milk and comparable infants fed a cow milk formula, suggesting that IgA in milk is not absorbed from the intestine in significant amounts.