Fatty acids, especially polyunsaturated fatty acids (PUFAs), inhibit a number of lymphocyte functions, including proliferation, cytokine production and cytotoxicity, but their mechanism of action is not known. This study investigated whether fatty acids inhibit lymphocyte proliferation by leading to the production of lipid peroxides, which are known to inhibit the growth of cells. The so-called "thiobarbituric acid-reactive substances" (TBARS) and lipid hydroperoxide contents of lymphocytes (0.75 +/- 0.04 and 1.30 +/- 0.39 nmol/mg protein in fresh cells, respectively) were increased by 48 h culture to 0.96 +/- 0.14 and 3.23 +/- 0.47 nmol/mg protein, respectively. The TBARS content was increased by culture in the presence of 100 microM PUFAs to between 1.46 +/- 0.11 (linoleic acid) and 2.39 +/- 0.31 (docosahexaenoic acid) nmol/mg protein. The lipid hydroperoxide content was increased by culture in the presence of 100 microM PUFAs to between 11.65 +/- 1.12 (linoleic acid) and 22.24 +/- 1.26 (docosahexaenoic acid) nmol/mg protein. These increases were partially prevented by inclusion of 10 microM vitamin E in the culture medium. Vitamin E (1 or 10 microM) enhanced concanavalin A-stimulated rat lymphocyte proliferation by approximately 45%. Vitamin E (10 microM) increased human lymphocyte proliferation by 35%. However, vitamin E did not prevent the inhibitory effects of fatty acids upon lymphocyte proliferation. It is concluded that inhibition of lymphocyte proliferation by fatty acids is not caused by their conversion to peroxidised products.