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methionine (SAM), in quantities up to 3 Amoles per 100 ml of medium, as equivalent sources of purine for cell growth, but not methylthioadenosine or S-adenosylhomocysteine. Utilization of SAM for growth was inhibited by the presence of L-methionine in quantities greater than 0.6 ,mole per 100 ml of medium. However, 6 ,umoles of L-methionine had no effect on growth when adenine or hypoxanthine was the source of purine. These sources also reversed the inhibitory effects of 6 ,umoles of the amino acid on the utilization of SAM. The presence of 400 ,umoles of the amino acid resulted in some inhibition of growth when the organisms were grown with adenine, hypoxanthine, or adenine plus SAM but had no effect on the total uptake of adenine-8-'4C. Studies on the uptake of radioactivity from a mixture of SAM-adenine-8-'4C and 8H-labeled SAM-methyl indicated that these components were taken into the cells at different rates which were altered by the presence of L-methionine. The fixation of I'S from "5S-labeled adenosylmethionine into the cells was inhibited by the presence of the amino acid. The cells synthesized and accumulated SAM in the presence of 400 ,umoles of L-methionine plus adenine even when exogenous SAM was supplied. Approximately 47% of radioactivity fixed from exogenous SAM-adenine-8-'4C and 12% from 8H-labeled SAM-methyl were found in reisolated SAM. Yall (12) reported that an adenineless mutant of Saccharomyces cerevisiae (SC-10-80-3-5) and its parent strain (SC-10) accumulated relatively large amounts of S-adenosylmethionine (SAM) when grown in a synthetic medium containing 400 0,moles ("excess" amounts) of L-methionine

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