The effects of feeding lipids protected against microbial degradation in the rumen, on the metabolism of β-carotene and cholesterol in the blood and milk of cows were studied. The diets fed to the cows consisted of a basal mixture of crushed oats and lucerne hay with a protected vitamin supplement containing α-tocopheryl acetate and β-carotene fed in conjunction with either (i) protected sunflower oil-seed rich in linoleic acid (PO), (ii) protected tallow (PT), or (iii) formaldehyde-treated casein (C) as a control. Diets PO and PT raised the concentrations of β-carotene and cholesterol in the blood plasma over that observed for diet C. Milk cholesterol concentrations were not affected by dietary supplements, but the level of β-carotene in milk of cows on diet PO showed a tendency to fall compared with milk from cows fed PT or C. The properties of the high density lipoprotein (HDLP) of the blood plasma which contained the β-carotene were affected by the PO diet. As a result of feeding this diet, the fatty-acid composition of the HDLP was altered and it emerged from a gel-filtration chromatographic column earlier than the control. This change in chromatographic behaviour was used as a measure of the effect of the diet, which, for some cows, was apparent long after the diet was changed. It is suggested that the altered lipid composition resulting from the PO diet affected the distribution of particle sizes of the HDLP and might interfere with the transfer of β-carotene from plasma to milk.