The translocation of amethopterin (MTX)* across the tumor cell membrane plays an important role in determining the cytotoxic effectiveness of this agent.’This has provided a stimulus for the investigation of the transport properties of the folate analogues in malignant This report describes studies from this laboratory which characterize a number of aspects of MTX transport in L12 I0 leukemia cells, Ehrlich ascites tumor cells, and rabbit erythrocytes. Other studies are presented which relate these findings to the transport characteristics of the naturally occurring folatcs.