Cell yields from human epidermal cell cultures 6 days after treatment with 0.1 ~tg/ml or 0.3 ~tg/ml 8-methoxypsoralen and 0.34 J/cm 2 longwave UV-radiation were significantly lower than nontreated controls (p < 0.025, p < 0.005 respectively) and significantly different from each other (p < 0.025).