Stock cultures of Blastomyces dermatitidis, Candida albicans, Cryptococcus neoformans, Histoplasma capsulatum and Sporothrix schenckii were grown in yeast phase on Sabouraud dextrose agar. Standardized suspensions (10(8) colony forming units/ml) were made and 1. the organisms treated for various times with 10% formalin/acetate buffer; 2. aliquots injected into the tail veins of Swiss-Webster mice. Minimal fungistatic and fungicidal concentrations of formalin were also determined. Five minutes or more of formalin treatment killed all organisms, in vitro, except C. albicans which withstood the 5-minute but not the 30-minute treatment. All fungi, except H. capsulatum which was not recovered, were cultured from mouse heart, kidneys, spleen, lungs and liver prior to but not after 5-minute formalin treatment. All yeasts were susceptible to concentrations of less than 1% formalin. These results emphasize the extreme fungicidal activity of 10% formalin and the uselessness of culturing even very briefly-fixed tissue.