Summary Metabolism of glycine and serine in Candida guilliermondii (Cast.) Lang. et G. 1)Cultures of Candida guilliermondii were supplied by replacement technique with l-14C-glycine, 2-14C-glycine, 1,2-14C-glyoxylate, U-14C-L-serine, 3-14C-L-serine, and 14C-formaldehyde. After increasing feeding periods up to 6hr the incorporation of radioactivity into components of 80% ethanol extract, nucleic acids, and protein fraction was determined.2)In C. guilliermondii the existence of an "one-carbon compound" pool was demonstrated. Carbon-14 from 14C-formaldehyde is preferentially introduced into the nucleic acid purines adenine and guanine, and into the amino acids serine and histidine. Radioactivity of 3-14C-serine is incorporated into purines, histidine, and methionine. Glycine C-2 also appears in the "one-carbon unit" pool, and significantly labels serine C-3. In addition, 14C-glycine is directly incorporated into serine.3)Synthesis of serine takes place by condensation of glycine with an "one-carbon compound". The glycine-serine conversion in C. guilliermondii is reversible. Radiocarbon from U-14C-serine s incorporated into glycine of ethanol soluble extract and of protein fraction.4)Serine hydroxymethyl transferase (E.C. 2.1.2.1) is present in a crude extract (0,1M phosphate buffer, pH 6,5) of C. guilliermondii. It was found a specific activity of 155 units/mg protein (enzyme activity assay: determination of absorbance change at 290nm by the formation of N5, N10-methylene tetrahydrofolic acid).5)Partial degradation of serine (radioactive serine was eluted from the chromatograms, diluted with inactive carrier, and chemically degraded with periodate) demonstrate also the preferential fixation of glycine C-2 and HCHO into serine C-3 atom.6.1,2-14C-glyoxylate is converted into glycine and serine.7.14C-serine is incorporated into cysteine and pyruvic acid. The conversion to pyruvate leads to labeling of alanine and phenylalanine after incubation with radioactive glycine or serine.8.Relatively small amount of radioactivity was found in glucose and saccharose after glycine or serine application.