[14C]-labelled palmitic acid (PA), oleic acid (OA), linoleic (LA) and arachidonic (AA) acids were transferred from macrophages (M phi) to lymphocytes (LY) when equal numbers of the two cell types were co-cultured. The relative degree and amounts of the fatty acids transferred from M phi to LY are as follow: AA (368.57 +/- 21.62) = OA (274.52 +/- 15.41) > LA (42.11 +/- 8.31) = PA (36.53 +/- 2.45). The transfer units are nmol/10(10) M phi/10(10) LY and the values are mean +/- SEM for 7 experiments. The [14C]-radioactivity transferred was mainly directed to the phospholipid fraction of the lymphocytes (85% by PA, 86% by LA, 83% by OA and 79% by AA). In the same order as above, phosphatidylcholine was the phospholipid moiety most heavily labelled (82% by PA, 71% by LA, 66% by OA and 47% by AA). The amount of [14C]-radioactivity transferred to stimulated lymphocytes of thioglycollate treated animals remained unchanged for LA, PA and AA but reduced for OA (71%). The significance of these observations for the immune functions of the cells and resolution of the question of whether some of the [14C]-isotope transfer involves a component of exchange or is unequivocally net fatty acid mass transfer are still being investigated.