The incubation of K562 cells with adriamycin resulted in a decrease in cell membrane fluidity as measured by electron spin resonance using the paramagnetic probe 5-doxylstearic acid. Coincidently, the antiproliferative effect of adriamycin was progressively inhibited as the concentration of adriamycin was increased. The results indicate that adriamycin induces changes in the plasma membrane of K562 cells after exposure to a low level of this agent.