Hexamethylenebisacetamide (HMBA), a potent inducer of differentiation of transformed cells such as murine erythroleukemia cells, causes a prolongation of the G1 phase of the cell cycle during which commitment to terminal differentiation is first detected. Removal of HMBA prior to the G1 phase aborts commitment. To further define the relationship between the G1 phase and commitment to differentiation, we used two inhibitors of cell cycle progression: aphidicolin, which blocks cells at the G1/S interphase, and deferoxamine, which blocks cells at an earlier stage during G1. HMBA-induced prolongation of G1 is associated with the accumulation of underphosphorylated retinoblastoma protein, decrease in cyclin A protein levels, and commitment to differentiation. G1 arrest of murine erythroleukemia cells induced by aphidicolin or deferoxamine is not associated with accumulation of under-phosphorylated retinoblastoma protein, suppression of cyclin A protein, or commitment of cells to terminal differentiation. Neither of the cell cycle inhibitors alters the effect of HMBA in inducing the G1-associated changes or commitment to differentiation. Taken together, the present findings indicate that the site of action of HMBA which leads to commitment is in a stage of the G1 phase prior to the point of cell cycle block caused by deferoxamine or aphidicolin. HMBA appears to cause cell differentiation with suppression of cell cycle progression by an action that affects events required for cell progression through G1, including accumulation of underphosphorylated retinoblastoma protein and changes in regulation of cyclin levels.