These studies are concerned with "mapping" the temporal order of the precommitment events in the differentiation pathway of the Friend erythroleukemia cell. We have used a single-block procedure in which a differentiation-specific inhibitor of a temperature-sensitive (ts) differentiation-defective mutation was used to block the differentiation program. Later, the block was removed, differentiation was allowed to proceed, and the time required to reach a reference marker was monitored. These studies have indicated that the mutations tsC2GP1 and tsB5A, the poly(ADP-ribose) polymerase inhibitor 3-aminobenzamide, and the glucocorticoid hormone dexamethasone blocked functions which are required just prior to commitment. We have also used a double-block procedure involving two consecutive restrictive conditions, which suggests that the 3-aminobenzamide- and tsC2GP1-blocked functions constitute a part of a sequentially ordered pathway leading to terminal differentiation. The convergence of the 3-aminobenzamide, dexamethasone, and ts mutational blocks just prior to commitment suggests that the blocked functions may be part of a major control mechanism for commitment. In these studies, we have introduced a cytochalasin B-based assay to monitor commitment. The use of cytochalasin B permits a direct assay for commitment and obviates the need for colony-forming assays using semisolid medium, which have inherent problems such as efficiency of plating.