Candida albicans generally grows in hyphae form in RPMI1640 medium. However, addition of 1,4-diamino-2-butanone (DAB), a competitive inhibitor of ornithine decarboxylase, decreased the amount of polyamines in C. albicans, and induced the proliferation of the C. albicans yeast form. The expression of CYR1 mRNA was significantly inhibited by the addition of DAB compared with that of the control. The amount of intracellular cAMP was also decreased by the addition of DAB. A specific adenylate cyclase inhibitor, cis-N-[2-phenylcyclopentyl]-azacyclotridec-1-en-2-amine (MDL-12,330A) promoted the growth of the yeast form. These results indicated that polyamines exist upstream of the adenylate cyclase-cAMP signal pathway and regulate the transformation of C. albicans.