We have studied the interactions among a varied group of neutral amino acids for uptake into two different animal tissue culture cell lines. 3T3 mouse cells and Chinese hamster ovary cells were used in these studies. The results showed that the affinities of the amino acids clustered into two groups, one including amino acids such as glycine, alanine, and alpha-aminoisobutyric acid and the other group included amino acids such as leucine and phenylalanine. The specificities and properties of these systems are similar to those described earlier for the Ehrlich ascites cell, System A (alanine-preferring) and System L (leucine-preferring). System A is highly concentrative, sensitive to sodium ion, and serves only poorly for exchange while System L is not very concentrative or sensitive to alkali metal ion, although it serves very well for exchange with endogenous amino acids. The kinetic parameters for several amino acids are reported and other properties of the transport systems are discussed.