Previous studies suggested that hepatic lipidosis caused by cationic amphiphilic drugs in rats is related to the capacity of these drugs to concentrate in liver lysosomes. These drugs inhibit lysosomal phospholipases, causing phospholipid accumulation. Amantadine, an inhibitor of influenza A virus replication, is a cationic amphiphilic drug which concentrates in the lysosomes of the Madin Darby canine kidney (MDCK) cell. In the present study, amantadine and chloroquine were shown to inhibit soluble lysosomal phospholipases isolated from MDCK cell in vitro. Both amantadine and chloroquine concentrated in MDCK cell lysosomes. These drugs caused phospholipid storage in cultured MDCK cells, and the amounts of the respective agents required to cause phospholipid storage correlated with the capacities of the agents to inhibit lysosomal phospholipases. The mechanisms involved in this phenomenon are discussed, and a three-step hypothesis is presented predicting which agents will cause phospholipidosis.