Human colostrum and milk collected at different times after the onset of lactation from 200 subjects were tested for the levels of IgG, IgA, and IgM classes of immunoglobulins. The technique of radial immunodiffusion was employed. The levels of IgG immunoglobulin ranged from 1.4 to 4.9 mg/gm of protein at different intervals after onset of lactation. No significant change was observed in the individual levels of IgG over 180 days of sequential testing. On the other hand, highest levels of IgM and IgA immunoglobulins in the colostrum and milk were observed during the first three to four days postpartum. The IgM levels ranged from 27 to 30 mg/gm protein and IgA levels, from 22 to 35 mg/gm protein. A 3- to 4-fold decline in the levels of IgM and IgA immunoglobulin was demonstrated in milk samples collected 15 to 180 days postpartum. These observations provide reference data on the levels of immunoglobulins in colostrum and milk at different stages of lactation.