An alteration in the rate of calcium transport appears to be the rate-limiting event for the commitment of murine erythroleukemia (MEL) cells to initiate a program of terminal erythroid differentiation. The dimethyl sulfoxide (DMSO)-induced commitment of MEL cells to erythroid differentiation can be inhibited by treatment of cells with the calcium-chelating agent EGTA. Upon removal of EGTA, cells initiate commitment without the 12-h lag normally observed after treatment with DMSO alone. Treatment of cells with DMSO in the presence of calcium ionophore A23187 causes cells to initiate commitment from time zero with no lag. These results suggest that the lag is the time required for DMSO to alter the calcium transport properties of the cell.