Styrene is one of the most important organic chemicals in use today. The highest human exposures to styrene take place by inhalation during the production of fibreglass-reinforced plastics. Styrene is oxidized by hepatic cytochrome P450 to styrene-7,8-oxide (SO), an epoxide that has been shown to induce chromosome aberrations, sister chromatid exchanges and micronuclei in many cell systems. In this work, the effect of SO on the expression of some genes involved in the cell cycle and apoptosis regulation in human white blood cells was studied. Lymphocyte cultures from four donors were exposed to 50 and 200 microM SO, 1% DMSO being the control. Aliquots of the cultures were taken at six different time points (30, 36, 42, 48, 60 and 72 h), total mRNA was extracted in each one of them and RT-PCR was carried out to analyze the expression of the genes p53, p21, bcl-2 and bax. Moreover, a cytokinesis block assay was performed to estimate cell proliferation kinetics by calculating the cytokinesis block proliferation index (CBPI), and to evaluate the number of cells undergoing apoptosis. Furthermore, apoptotic events were detected by the DNA fragmentation assay. In our results, a high interindividual variation in the expression of the studied genes was observed. Expression curves obtained for the four genes, together with the data from the CBPI and apoptotic cells scored, suggest that exposure to high levels of SO may induce a delay in the cell cycle, probably directed to allowing repair systems to act on the genotoxic damage produced, more than driving cells towards programmed cell death.