Both saturated and unsaturated fatty acids (FA) bound to albumin inhibit phytohaemagglutinin (PHA) transformation of human lymphocytes cultured in fetal calf serum (FCS). With two unsaturated FA inhibition is maintained, however, when saturated and unsaturated FA are added simultaneously the inhibition is abolished. When lymphochtes are cultured in human AB serum, fatty acids neither inhibit nor stimulate PHA transformation. The reasons for the difference between FCS and human serum are discussed. These results argue against reports that unsaturated FA plays an immunoregulatory role in humans.