The cells of Saccharo-myces cerevisiae were grown in a stock medium in which the following factors could be varied: sugar supply, O2 tension, and temp. Results showed that a maximum rate of radiophosphate (P*) transfer across the yeast-cell membrane was dependent upon having an abundant carbohydrate supply and a temp. of 37[degree] C. The rate of diffusion of P* into the cell in the absence of carbohydrate is only about 2 X 10-2 [image]/Kg. yeast/hr.