Evidence has been obtained for the formation of a new molecular species of the enzyme dihydrofolate reductase in an amethopterin-resistant strain of the mouse lymphocytic leukemia L4946. The evidence consists of a more rapid elution of the R enzyme on Sephadex G-100, a greater reactivity with the R antibody, a marked degree of activation by C1-, whereas, the S enzyme is not affected, and also activation of the R enzyme by low concentrations of PCMB.