A new direct spectrophotometric assay for dihydrofolate reductase has been developed. The reaction is observed at 282 mµ and is based on the disappearance of dihydrofolate. Based on this assay, the values for normal leukocytes are 0-5 mµmoles of dihydrofolate reduced to tetrahydrofolate, per mg. of protein per hour at 28 C. The respective values for chronic myelogenous leukemic granulocytes are 25-67, chronic lymphocytic leukemia 0-18, acute lymphocytic leukemia 7-19, and for leukocytes of leukemoid reaction 4 to 5.

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