Amphotericin B (AmB) and filipin effects on L and HeLa cells were compared by monitoring drug-induced potassium leakage from cells, changes in radioactive uridine incorporation into cellular ribonucleic acid, protein leakage from cells, and cell viability. L cells were much more susceptible to both AmB and filipin than were HeLa cells, but the overall dose response was similar. For AmB, the various effects were easily separable. Potassium leakage occurred at the lowest concentrations of AmB and was reversible. Inhibition of uridine incorporation and loss of viability occurred at intermediate levels, and protein loss occurred at higher levels. In contrast, filipin was much more potent; its effects on potassium leakage were only minimally reversible, and the separation of the permeabilizing effects from complete cell lysis was possible only over a limited concentration range and for a short time.