Background : The erythemal response and delayed tanning of the skin to UV radiations which are used as diagnostic phototest and guideline of phototherapy are different according to the races and light sources. Objective : The MED and MMD induced by UVB radiation were measured in 130 normal young adult Koreans. Methods : In this study, a high pressure mercury are lamp(Burdick UV-800) and a sunlight fluorescent lamp(Waldmann UV 800) were used as UVB light sources. Multiple sites of the lower baek or buttock skin were irradiated with an increasing dose by a constant, anount. The minimal doses of erythema response and delayed tanning of the skin were assessed visaually at 24 hours and 7 days after irradiation, respectively. Results : MED and MMD of Burdick UV-800 are 18.0+9.8mJ/cm(mean+S.D.), 29.7+12.5mJ/cm, respectively. MED and MMD of Waldmann UV 800 are 70.8+28.5mJ/cm and 91.2+33.1mJ/cm, respecively. The most frequent MED by Burdick UV-800 is 10mJ/cm and those by Waldmann UV 800 are 50m J/cm and 60m J/cm. Conclusion : In this study MED and MMD of young adult Koreans by two kinds of UVB light sources were assessed. The results : how that MED is less than MMD and the levels of MED under which the photosensitivity is suggested are 5.6mJ/cm by Burdick UV-800 anr 32.4mJ/cm by Waldmann UV 800. (Kor J Dermatol 1994; 32(2): 253-257)