A rat heterotopic cardiac transplant model was used to study the effect of dietary lipids on the immune response. Animals receiving linoleic acid (LA), oleic acid (OA), and fish oil (FO) enriched diets showed significant prolongation of allograft survival when compared to the control diet fed animals. When LA was given to animals who had received a single donor specific transfusion (DST) augmentation of the beneficial DST effect was observed, while the OA and FO fed groups showed no differences from control DST animals. Dietary regulation of the immune response, possibly through manipulation of arachidonic acid metabolism, is implied.