The concentration of the anticarcinogenic fatty acid, 9c, 11t-18:2, in human milk was determined by gas-liquid chromatography (GLC). The mean concentration of 20 samples from 5 women taken at 1, 7, 14, and 21 days was: 0.18% +/- 0.02; range, 0.14-0.28%. Identity was confirmed by GLC-mass spectrometry (MS). Conjugated isomers other than 9c, 11t-18:2 were not detected. The amounts were not changed by supplementation of the maternal diet with fish oil beginning on day 1 after the milk sample was taken.