A new mutation introducing a one-carbon requirement (e.g., formate) for the glycine-supplemented growth of a serine-glycine auxotroph (ser1) was correlated with a lack of glycine decarboxylase activity. The presence of oxalate decarboxylase activity or glyoxylate decarboxylase activity did not overcome the one-carbon requirement. Another mutation characterized by the absence of oxalate decarboxylase activity did not introduce a one-carbon requirement. The presence and physiological significance of glycine decarboxylase activity in Saccharomyces are thus inferred.