The reduction of hydroxypyruvate by enzymes of human origin is described and is shown to proceed at a faster rate than that of pyruvate. Maximal reaction rate conditions for the reduction of hydroxypyruvate by normal and pathological human sera have been determined at 37° C and the temperature-activity relationship studied. Confirmation of the reverse reaction has been obtained and tentatively suggests that the reduction is an index of lactate dehydrogenase activity.