There is now considerable evidence that polyunsaturated fatty acids (PUFA) are effective in vitro at limiting the growth of cancer cells while not affecting normal cells to the same extent. Twenty carbon PUFA, especially of the n3 series, have been shown to be particularly potent; while the eighteen carbon n6 fatty acid, linoleic acid has been implicated in growth stimulation of breast cancer. We report here on the comparative effects of a range of eighteen and twenty carbon fatty acids of varying degrees of unsaturation on normal and transformed fibroblasts in culture. All moieties of the n3 series showed high potency in limiting transformed cell growth, while cis and trans monounsaturates and pre-delta-6-desaturation n6 polyunsaturates induced a mixed response, even inducing comparative growth stimulation with some fatty acid concentrations.