The human pathogenic, dimorphic fungus Candida albicans produces in vitro a factor designated morphogenic autoregulatory substance (MARS) which suppresses yeast to mycelial transition. MARS is best produced when a high concentration of yeast cells (2 X 10(8)/ml) in a chemically defined medium is incubated at 37 degrees C for 9-15 h. MARS was isolated to apparent homogeneity by the following successive steps: charcoal absorption; pyridine elution; ether extraction; high pressure liquid chromatography, and Sephadex LH-20 chromatography. A 469-fold increase in purity was obtained by these procedures. The use of purified MARS to study C. albicans yeast to mycelial transition may lead to a better understanding of morphogeneis of this important opportunistic pathogen.