Samples of breast milk and serum were taken from 29 women at various stages of lactation before and after they had ingested 1 raw egg and half a pint of cow's milk. The samples were analysed for cow's milk and egg proteins using a solid-phase radioimmunoassay. beta-Lactoglobulin, ovalbumin (OA) and ovomucoid were detected in breast milk from 10 out of 19, 13 out of 22 and 7 out of 9 women, respectively, in concentrations ranging from 110 pg/ml to 6.4 ng/ml. Maximum levels in breast milk were attained 4 or 6 h after ingestion and in serum 1-2 h earlier. The OA in breast milk was of normal molecular size (43,500 daltons) and indistinguishable from native OA in the radioimmunoassay. Gel filtration of whole breast milk or high molecular weight fractions at pH 2.6 failed to reveal additional OA, suggesting that immune complexes were not present. In serum, OA was detected both in its native form and in immune complexes. The possible significance of these antigens in the suckling infant is discussed.