Supplementation of the culture medium of Candida utilis (Torulopsis utilis) and of Saccharomyces cerevisiae with L-methionine results in greatly increased formation and accumulation of S-adenosylmethionine (Schlenk and DePalma, 1957a), a sulfonium compound whose structure comprises adenosine and methionine in the following way: Adenine-ribose-S-CH2 CH2 CH(NH2) C OOH &H3 Under suitable conditions, a concentration of 40 to 60 Amoles per gram of yeast cells on a dry weight basis may be obtained. Because the molecular weight of the sulfonium ion is 399, this corresponds to a concentration of 1.6 to 2.4 per cent. S-Adenosylmethionine has the typical absorption spectrum of an adenosine compound with Em 15,400 at 260 m,u. Its concentration remains essentially constant after growth and metabolism have ceased and the quantity of nucleic acids has declined. It appeared possible, therefore, to determine the location of S-adenosylmethionine within the cells by ultraviolet microscopy. As a preliminary to the optical studies and for correlation, the earlier chemical studies had to be amplified. The present report is a detailed account of our observations which have been summarized earlier (Svihla and Schlenk, 1958). MATERIALS AND METHODS Candida utilis (Torulopsis utilis) strain ATCC 9950 was used in the present experiments. The composition of the growth medium, culture conditions, and harvesting procedure have been described earlier (Schlenk and DePalma, 1957b). L-Methionine in a concentration of 4 to 6 ,umoles per ml, or a combination of S-methyl-L-methionine and L-homocysteine (4 ,umoles of each) was used as organic sulfur supplement. For