5,10-Dideazatetrahydrofolic acid (DDATHF) is a folate antimetabolite that shows activity against glycinamide ribonucleotide (GAR) transformylase, a folate-requiring enzyme in the de novo purine nucleotide biosynthetic pathway. Previous studies from our laboratory have shown that DDATHF is an effective inducer of the maturation of HL-60 promyelocytic leukemia. In solution, DDATHF is a mixture of two diastereomers due to an asymmetric configuration at carbon 6. Incubation of HL-60 cells with 1 microM of each diastereomer resulted in an inhibition of cellular proliferation after 48 h that preceded an increase in the number of differentiated myeloid cells, as determined by the ability of cells to reduce nitroblue tetrazolium (NBT) and by the binding of the myeloid-specific antibody Mo 1. Several analogs of DDATHF were also tested as inducers of the differentiation of HL-60 cells. With the exception of the 10-acetyl analog of 5-deazatetrahydrofolic acid, all compounds displayed similar activities as inducers of maturation. The finding that both stereoisomers of DDATHF, as well as the analogs tested, could selectively reduce intracellular purine nucleotide levels suggested that these compounds inhibited purine nucleotide biosynthesis de novo. This possibility was confirmed by the finding that hypoxanthine completely prevented the reduction of intracellular purine nucleotide levels, as well as the induction of differentiation and the inhibition of cellular growth, by these folate analogs. The results suggest that GAR transformylase is a target for a series of compounds whose structures resemble that of tetrahydrofolate and indicate that the inhibition of GAR transformylase by these compounds is sufficient to induce the maturation of HL-60 leukemia cells.