A formaldehyde-treated casein-safflower oil supplement was given at levels of 0, 75, and 150 gjday to lambs receiving 600 gjday of a diet of equal parts chopped lucerne hay and crushed oats. The response of the lambs was studied in terms of food intake and liveweight gain, the chemical composition of the body, the incorporation of linoleic acid into body fat, the digestibility of the diets, and the levels of urea, a-amino nitrogen, and glucose in blood plasma.The supplemented lambs appeared to discriminate against the supplement to a small extent. There was no detectable effect of the supplement on the gross chemical composition of the body but the linoleic acid in the supplement was extensively incorporated into body tissues; the progress of incorporation was described by a curve of diminishing increments. Approximately half the linoleic acid ingested was retained in the body. The supplement was readily digested and plasma levels of urea, a-amino nitrogen, and glucose were increased in the lambs given 150 gjday of the supplement.