The synthetic "C" nucleoside, tiazofurin (2-beta-D-ribofuranosylthiazole-4-carboxamide), its selenium analogue selenazofurin, and the related inhibitor of inosine 5'-phosphate (IMP) dehydrogenase, mycophenolic acid, are effective inducers of the terminal differentiation of HL-60 promyelocytic leukemia cells. The inhibition of cellular replication and the induced maturation produced by these agents appears to be a consequence of the inhibition of IMP dehydrogenase, since growth inhibition is partially reversed and differentiation is completely prevented by the simultaneous exposure of cells treated with inhibitors of IMP dehydrogenase to exogenous guanosine, which serves to circumvent the effects of the blockage of IMP dehydrogenase. The exposure of HL-60 leukemia cells to inhibitors of IMP dehydrogenase caused a marked reduction in the incorporation of [3H]mannose into both cellular glycoproteins and their lipid-linked oligosaccharide precursors; these effects are presumably due to the pronounced decrease in intracellular levels of guanosine triphosphate produced by blockage of IMP dehydrogenase. Maximum effects on glycoprotein biosynthesis occurred within 8 h of exposure to the inhibitors of IMP dehydrogenase. The simultaneous incubation of cells with guanosine and these inducers of differentiation partially prevented the reduction in [3H]mannose incorporation into glycoproteins, supporting a relationship between glycoprotein biosynthesis and guanosine triphosphate formation in the induction of differentiation by inhibitors of IMP dehydrogenase.