We demonstrated previously that pentoxifylline in millimolar concentrations can inhibit superoxide anion production by human spermatozoa. In the present study we have examined the effects of the same concentrations of pentoxifylline on experimentally induced lipid peroxidation, as measured by malondialdehyde formation in the thiobarbituric (TBA) assay. Under the experimental conditions used, preincubation of spermatozoa with pentoxifylline led to a significant dose-dependent stimulation (p < 0.005) of malondialdehyde production amounting to 10.77 +/- 2.35%, 13.45 +/- 2.99% and 17.4 +/- 1.99% (mean +/- SEM) for 1.9, 3.7 and 11.2 mmol/l pentoxifylline, respectively. In the presence of 11.2 mmol/l pentoxifylline, an increase in iron-catalysed lipid peroxidation potential was detected in samples of spermatozoa from 29 infertile men, regardless of their initial levels of malondialdehyde. The results of this study indicate that pentoxifylline might further augment the ferrous ion-stimulated decomposition of pre-accumulation lipid hydroperoxides in the sperm plasma membrane and thus promote malondialdehyde generation in the TBA assay. It is concluded that the stimulatory effect of pentoxifylline on iron-induced lipid peroxidation may have an adverse effect on the quality of sperm suspensions prepared for in vitro fertilization, a possibility which should be investigated further.