The plasma lactic dehydrogenase (LDH) activity was increased in RF mice with primary and transplanted radiation-induced myeloid leukemia. The degree of elevation in mice with transplanted leukemia varied irregularly with the time after transplantation and with the transplant generation, the mean of all values being about 14 times normal. The extent of LDH increase in any transplant generation was correlated with the severity of leukemic infiltration of the liver and other organs. This correlation indicates that LDH mirrors the severity of the leukemic process. Whole-body x-radiation in the absence of transplanted leukemia, also caused a transient elevation of plasma LDH.