An active system for concentrating tyrosine by the Sarcoma 37 ascites cells has been demonstrated. This transport system was affected by metabolic inhibitors and subject to competitive inhibition by several amino acids. No stereospecificity for tyrosine was observed; however, certain structural requirements for concentration were demonstrated.

A stimulation in concentration of aromatic amino acids by p-fluoro-dl-phenylalanine was observed. The stimulatory effect was characteristic for the fluorinated phenylalanines, since no other analogs produced this effect. Cellular concentration by tyrosine was not affected by certain alkylating agents.

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