Candida albicans was established in large numbers throughout the gut after one oral challenge in the germ-free and in the conventional mouse. Of the strains tested, only the germ-free ND 1 mouse appeared to be susceptible to infection, and this was confined to the stomach mucosa; lesions contained large numbers of hyphal and mycelial forms with blastospores. These forms were also seen in the gut of resistant germ-free ND 4 mice after challenge. Only budding yeast forms were seen in the gut contents from conventional animals. The concentration of sulfhydryl-containing compounds was decreased in the stomach contents from germ-free mice. The stomach tissue of conventional animals seemed to be more acidic than that of germ-free animals, and association of C. albicans with conventional mice neutralized some of this acidity. E(h) values of contents from the gut of unchallenged mice were usually higher in conventional than in germ-free animals; after challenge, the E(h) in both groups decreased. Some reciprocal effects of intestinal microorganisms and host are discussed in relation to intestinal candidiasis.