SUMMARY Growth of four fungi on a medium of potato tuber extract and glucose was considerably higher than on a defined medium of glucose, asparagine, mineral salts, biotin and thiamine. Modification in turn of the carbon, nitrogen and growth factor components of the defined medium to resemble the extract medium did not indicate that any singlecomponent was responsible for growth stimulation; stimulation appeared to be due to the combined effects of all components. A synthetic medium of glucose, citric acid, amino acids, mineral salts and growth factors resembled the extract medium in composition and approached it in the ability to support growth of the test fungi.