Polyethylene glycol (PEG 8000) is a potent cancer chemopreventive agent. This osmotic laxative polymer markedly suppresses colon cancer in rats. To explain the mechanism, we have tested the in vitro effect of PEG on four human cell lines. Two poorly differentiated adenocarcinoma lines (HT29 and COLO205), a fetal mucosa line (FHC) and a differentiated line (post-confluent Caco-2) were incubated with various PEG concentrations for 2-5 days. Results show that PEG markedly and dose-dependently inhibited HT29 and COLO205 cell growth. This cytostatic effect was associated with a blocking of the cell cycle in G0/G1 phase. In addition, PEG decreased the viability of HT29 and COLO205 adenocarcinoma cells. In contrast, post-confluent intestinal-like Caco-2 cells and normal FHC cells were, respectively, not or little affected by PEG. Moreover, the lactate concentration increased twofold in the medium of PEG-treated HT29 cells compared with untreated cells. Microscopic observations showed that PEG induced cell shrinking, membrane blebbing and the condensation of nuclear chromatin. However, because no DNA ladder and no annexin staining were detected, we presume that PEG did not induce apoptosis. PEG increased the osmotic pressure of the culture medium. Hyperosmotic media with added NaCl or sorbitol also inhibited HT29 cell growth, and increased lactate release. These results suggest that PEG may be selectively cytostatic for proliferating cancer cells. This growth inhibition may be due to the high osmotic pressure induced by PEG in vitro. Because the osmotic pressure is high in feces of PEG-fed rats, it may explain the suppression of colon carcinogenesis by PEG.