Both saturated (SFA) and unsaturated fatty acids (UFS) inhibited PHA- and PPD-induced lymphocyte transformation. The degree of inhibition varied with the nature of the fatty acid (FA) and further comparison with their effect on unstimulated lymphocyte cultures suggests a specific action of polyunsaturated fatty acid (PUFA) on the lymphocyte PHA- and -PPD interaction. These findings are briefly discussed in the light of similar observations with PUFA-derived prostaglandins.