Ruminants have extremely low concentrations of polyunsaturated acids in their adipose tissue as a result of hydrogenation of dietary lipids by microbes in the rumen. The possible association between intake of ruminant products high in ‘saturated’ lipids and the incidence of atherosclerosis in man has stimulated research on procedures for altering the fatty acid composition of ruminant tissue lipids. Vegetable oils, emulsified with a protein, spray-dried, and treated with formaldehyde, have been found to be protected from hydrolysis and hydrogenation in the rumen (Cooket al.1970; Scott, Cook & Mills, 1971). Their subsequent digestion in the small intestine, and absorption of the fatty acids by the animal, results in tissue lipids high in linoleic acid (Faichneyet al.1972; Faichney, Scott & Cook, 1973; Cooket al.1972; Garrettet al.1976).