Feeding animals a diet high in linoleic acid for 7 days had no effect on the in vitro production of PGE2 by unstimulated macrophages. Feeding animals the high linoleic acid diet for 30 days greatly increased the production of PGE2 by macrophages when they were unstimulated, but decreased the production of PGE2 when they were stimulated with LPS. Feeding animals a diet high in linoleic acid for 30 days increased the production of TxB2 by macrophages when they were unstimulated, but decreased the production of TxB2 when they were stimulated. Normal, unstimulated splenic macrophages produced almost 80 times more TxB2 than PGE2. However, when the macrophages were stimulated the ratio decreased to six or less because of a greater increase in PGE2 production. The high linoleic acid diet did not inhibit the antibody dependent cellular cytotoxicity of macrophages.