The amount of TGF-beta contained in human whey was studied by the colony formation of NRK47F cells. It was noted that a factor inducing colony formation did exist in human whey, and its action was neutralized when anti-TGF-beta antibodies were introduced. This suggests that TGF-beta does exist in human whey. In colostrum, the total amount of TGF-beta was 1365.7 +/- 242.9 ng/ml, of which the active form comprised 728.1 +/- 248.7 ng/ml (n = 21). In late milk, the total TGF-beta was 952.5 +/- 212.6 ng/ml, with an active form of 178.7 +/- 157.3 ng/ml. Thus human milk contains a large amount of active TGF-beta. Furthermore, it was revealed by the reverse transcriptase polymerase chain reaction that mRNAs coding TGF-beta 1 and TGF-beta 2 exist in human milk cells. These results suggest that both TGF-beta 1 and TGF-beta 2 exist in human milk.