ABSTRACTThe dose rate effect of ultraviolet light (UV) was investigated from a view point of the law of reciprocity bothin vivoandin vitro. The reciprocity in UV erythema reaction was examined in seven excision repair deficient xeroderma pigmentosum (XP) patients with monochromatic UV at wavelengths of 255 nm and 290 nm using a 1% neutral filter (NF) to reduce UV intensity to 1/100. For thein vitrostudy, the reciprocity in UV cell killing was examined in four normal and three excision repair deficient XP fibroblast strains using varied intensities to 1/100 at 254 nm.As in the results obtained by Satoh et al. in normal subjects, the law of reciprocity did not hold for erythema reaction in XP patients at 255 nm when the dose rate was changed a hundred fold. In the XP patients, the minimal erythema dose (MED) required by 1/100 intensity was on the average 1.57 times higher for 255 nm and 1.12 times higher for 290 nm as compared with MED without NF. On the other hand, the law of reciprocity was found to hold for cell killing in both normal and XP cells by 254 nm UV at the intensity range of 1:100.