The relationship between the N-methylformamide (NMF)-induced differentiation of HL-60 human promyelocytic leukemia cells, to myeloid cells, and their proliferative potential was determined in suspension culture. Continuous incubation with 170 mM NMF induced 70% of cells to differentiate after 2-2.5 cell divisions, whether or not cells were replenished with fresh medium and serum. 100 mM NMF induced significant levels of differentiation only when cells were allowed to reach a plateau of growth, and this was prevented by replenishment of the medium and serum. Under these conditions of "re-feeding", the cells could be made to undergo greater than 10 cell divisions, and differentiated only when they were allowed to reach a plateau in their growth. The data suggests that NMF may induce differentiation only when the proliferative potential of the cells is limited, either by the drug itself or by the conditions of growth.