Blastospores of Candida albicans germinated in a proline-biotin-buffer medium incubated at 37 C. Certain other amino acids in the glutamate, aspartate, and pyruvate families also fostered germination but generally to a lesser extent than did proline. L-Cysteine, D-proline, and certain structural analogues of Lproline inhibited proline-stimulated germination. The concentration of phosphate and glucose was crucial to amino acid-stimulated germination of C. albicans. Clinical isolates and stock cultures varied in their response to the germ tube-inducing activity of proline or other amino acids. The proline-buffer medium cannot be used in a diagnostic test for production of germ tubes by isolates of yeasts.