The effect of the major phenytoin metabolite 5-(parahydroxyphenyl)-5-phenyl-hydantoin (p-HPPH) was studied on cultures on human fibroblast-like cells grown out from explanted gingival biopsies. The explants were taken from children undergoing phenytoin medication. The results showed that the number of cells per culture decreased whereas the protein and DNA- contents remained relatively unaffected. This effect was most pronounced at the concentrations of 0.20 /μg/ml p-HPPH. The results indicate that the metabolite interfere with cell division without affecting protein or DNA synthesis.