The serum LDH and its isoenzymes of 61 medical diagnostic X-rays workers were observed in this paper.,The results showed that both total activity of the LDH and LDH,3, increased clearly and LDH,1, decreased clearly when irradiated geoup compared with control group.,Comparing with control group, the LDH,1, of both,<,10 years and 10-20 years of occupatipnal lengh of service groups decreased markedly and LDH,3, increased markedly. The LDH,1, decreased and LDH,3, increased respectively with the length of service within 20 years, and both LDH,1, and LDH,3, returned to normal level when the length of service was more than 20 years.,Total activity of the LDH,1, and LDH,3, of both 20-50 mGy and,>, 50 mGy dose group increased markedly and LDH,1, decreased clearly when it was compared with control group. The changes of the LDH and its iscenzyme related to cumulative dose. Therefore, we consider that the changes of the LDH and its isoenzyme is a significant parameter to the radiation effect of low-dose.