The use of broad-spectrum antibiotics for preoperative preparation of the colon is attended in some cases by the overgrowth of yeasts of variable pathogenicity within the intestinal tract. Usually these organisms remain confined to the alimentary canal, but in debilitated or cachectic patients systemic infection may occur. The lesions produced by the proliferation of yeasts are seldom of magnitude sufficient to endanger life, but at the very least the convalescence of postoperative patients is prolonged. Consequently it is important to consider the addition of yeast inhibitors of the preoperative agents used for preparation of the large intestine, provided that attainment of this objective does not interfere with the mechanical and antibacterial aspects of such a preparatory regimen. The studies to be reported in this paper were carried out in an attempt to evaluate the effectiveness of one such agent, nystatin,* in suppressing the overgrowth of yeasts resulting from neomycin therapy.