Dihydroxyacetone (DHA) was found rapidly to enter human erythrocytes. When incubated with intact red cells, 14C-labeled DHA rapidly appeared in intermediate metabolites and in lactate. Crude hemolysates were found to phosphorylate DHA and glyceraldehyde (GA) but not glycerol. Fifty-fold purification of the phosphorlylating enzyme was achieved by removal of hemoglobin on CM-52 and Sephadex G-200 chromatography. During purification there was no change in the ratio of DHA to GA utilization. The enzyme had a pH optimum of 6,6, a Km of 0.5 µM for DHA, and a Km of 11 µM for GA. With GA as substrate, the Km for (ATP-MG)2- complex was 1.45 mM, and no cooperative interaction was observed. With DHA as a substrate, cooperative interaction with respect to (ATP-Mg)2- complex was found with an n value of 1.4 and a K of 0.44 mM. At temperatures between 15° and 40°C, the enzyme activity gave a linear Arrhenius plot, with deviation from linearity at lower temperature levels. The activity of the enzyme was 0.151 ± 0.011 U/g Hb in five normal subjects, and only slightly increased levels were observed in a patient with marked reticulocytosis. The DHA-phosphorylating activity of erythrocytes is attributed to the activity of triokinase (ATP: D-glyceraldehyde-3-phosphotransferase; E.C.2.7.1. 28). Although DHA is rapidly cleared from blood after injection into rabbits, substantial metabolism is possible for a prolonged time after administration because of the very low Km of the enzyme for DHA. The capacity of red cells to utilize DHA may prove to be useful in the maintenance of 2,3-diphosphoglycerate (2,3-DPG) levels during blood storage and in the in vivo manipulation of red cell 2,3-DPG levels.