Breast milk cell culture supernatants from atopic mothers (nineteen) were compared by ELISA with normal breast-feeding controls (twenty-one) for regulation of cord blood lymphocyte IgA and IgE secretion in vitro. A minority of atopic (five out of nineteen) and normal (three out of twenty-one) cell supernatants stimulated cord blood lymphocyte IgA release to the same extent. The others were inactive. Stimulation was not related to breast milk cells donor atopic history or cord blood lymphocyte atopic heredity. In contrast, 70% of atopic milk cell supernatants stimulated cord blood lymphocyte cultures to form IgE (means +/- s.d. = 2070 +/- 2240 pg/culture) while stimulatory supernatants (24%) from normal donors resulted in less lymphocyte IgE release (means +/- s.d. = 680 +/- 490 pg/culture) (p less than 0.001). These differences did not correlate with breast milk cell supernatant IgE concentrations, cord blood donor serum IgE levels or atopic heredity.