Abstract The ADP-ribose synthesis inhibitor 2-aminobenzamide was found to reduce the induction of haemoglobin synthesis in murine Friend erythroleukemia cells, cultured continuously for 96 h with 5 mM N′-methylnicotinamide, with over 50% inhibition at equimolar doses. 2-Aminobenzamide was optimally effective as an inhibitor of differentiation if present only during the time period 24–48 h after initial N′-methylnicotinamide exposure. A transient, genomewide DNA hypomethylation accompanies induction by N′-methylnicotinamide. Although 2-aminobenzamide does not seem to affect normal DNA methylation in cultured cells, the observed DNA hypomethylation in cells cultured for 24 h with 5 mM N′-methylnicotinamide was antagonized over 50% by equimolar 2-aminobenzamide. We suggest that ADP-ribosylation has a positive modulatory role in erythroid differentiation, and possibly in the process(es) leading to DNA hypomethylation following inducer exposure.