The long-term effects of feeding Holstein cows plant lipids protected from microbial hydrogenation in the rumen were studied. Of particular interest were cow health and changes in fatty acid and cholesterol concentrations of milk and meat. Safflower oil-casein or safflower oil-casein treated with formaldehyde to impede microbial attack were fed to two groups of three cows as 10% of the concentrate ration for two lactations. Production of milk fat of cows fed the protected concentrate increased significantly. Linoleic acid of milk fat was twice normal, providing a polyunsaturated milk. Cholesterol of milk or meat did not increase even though cholesterol of blood plasma was higher in both groups fed safflower oil than in control cows. Cardiovascular systems showed no marked abnormalities and no differences that could be due to treatment. All cows maintained normal health and milk production throughout the experiment.