The antitumor antibiotic adriamycin was found to be a potent modulator of the human erythrocyte discocyte echinocyte transition. Incubation of discocytes for 10 min with 10 microM adriamycin inhibited calcium-induced echinocytosis by 90 per cent. Adriamycin itself had no effect on erythrocyte morphology, a feature which distinguished it from other amphipaths which bring about the formation of a cupped cell morphology. Additionally, adriamycin differed from amphipaths such as the phenothiazines in that concentrations which prevented echinocytosis had no effects on calmodulin, as measured by effects on calmodulin-stimulated 45Ca2+ uptake into inside-out red cell vesicles. Adriamycin, paradoxically, appeared to cause a fall in the levels of erythrocyte polyphosphoinositides, but prevented further breakdown induced by calcium loading. This fall in inositides may be apparent rather than real, as the drug did not cause breakdown of the inositides to either inositol di- or triphosphates in red cell vesicles. Instead, it inhibited breakdown. It is possible that adriamycin may complex out the inositides and thus maintain levels of the inositide polyphosphates, congruent with the maintenance of the discocyte morphology. Interference with inositol lipid metabolism may be an important aspect of the pharmacology of adriamycin.