The effects of oleic acid (oleate) were examined using two in vitro model systems. In a concentration-dependent fashion oleate activated or inhibited the (Ca2++ Mg2+)-ATPase, or Ca2+pump ATPase, of membranes isolated from human red blood cells (RBC's). Concentrations of oleate which inhibited the Ca2+pump ATPase also inhibited the Na+-K+pump ATPase. Likewise, in a concentration-dependent fashion oleate increased or abolished ATP dependent45Ca2+transport into inside-out vesicles (IOV's) prepared from human RBC's. Addition of 500 μM oleate to IOV's which had already accumulated45Ca2+resulted in rapid loss of the ion. The effect was shown to be due to membrane disruption; a conclusion based on oleate-induced unmasking of latent acetylcholinesterase activity in IOV preparations. The results are compatible with, but do not prove, that membrane disruption caused by circulating free fatty acids and similar membrane active agents might play a role in the cellular injury associated with certain pathophysiologic states.