The effect of graded levels of dietary rumen-protected methionine (rp-Met) or lysine (rp-Lys) on the fatty acid composition of Comisana ewes' milk was investigated. The animals lambed in autumn and were separated from their lambs 7 weeks after parturition; they were divided into five groups of nine which were given different rations. The control group was fed on a mixture of vetch and oat hay with a pelleted concentrate; the other groups were given the control ration supplemented with rp-Met (3·5 or 7·0 g/kg) or rp-Lys (10·5 or 21·0 g/kg). Milk fatty acid composition was determined by gas chromatography, and these values were subjected to principal components analysis. A correlation was found between fatty acid composition and diet. We demonstrated that giving the experimental diets significantly affected milk fat, to an extent that was independent of the levels of the supplementary amino acids. The milk fat of the control group contained largely caprylic, capric and lauric acids, whereas that of the groups given the supplementary amino acids contained largely palmitic, palmitoleic, stearic and oleic acids. Thus supplementation resulted in an increase in the proportion of 16[ratio ]0–18[ratio ]3 (primarily palmitic and stearic) and a reduction in the proportion of 4[ratio ]0–12[ratio ]0 fatty acids. There was also a modest increase in the unsaturated: saturated fatty acid ratio.