Metabolic energy availability and proliferative activity were studied in HeLa cells treated with Adriamycin (Adr). Conditions (ID50) that induced growth delay, without significant cell lethality, did not appreciably alter ATP content, over 72 hours after treatment. Energy-dependent membrane permeability properties, assessed with the vital dye fluorescein, were similarly unaffected. The cytostatic effect was confirmed by the incorporation rate of 3H-thymidine and 14C-leucine, and by flow cytometric analysis of cell cycle progression. Early depletion in cellular ATP occurred when Adr induced an evident cytocidal effect.