Induction of immunoglobulin secretion by human colostrum was investigated using human peripheral blood lymphocytes (PBL) and Epstein-Barr virus transformed human B lymphoblastoid cells. Stimulation of the cells with colostrum induced IgA plaque forming cells but neither IgG nor IgM plaque forming cells, indicating the occurrence of IgA specific helper factor (alpha HF) in human colostrum. alpha HF activity was eluted into fractions with an apparent molecular weight of about 80 kD by gel filtration, and with a PI range of 5.8 to 6.2 by chromatofocusing. IgA secreted by PBL stimulated with alpha HF had a similar molecular weight distribution to that of IgA in human colostrum. From these results a hypothesis is proposed; IgA-committed B cells in the mammary gland differentiate to plasma cells producing dimeric IgA after stimulation by alpha HF so that the dominant immunoglobulin in human colostrum is IgA.