The occurrence of agglutinating and precipitating antibodies to Candida species was determined in 50 patients undergoing operations on the large intestine. The number of patients with oral or faecal yeast colonization and the amount of faecal colonization increased during the period after operation. No patient developed overt fungal infection but 15 patients developed precipitins to mannan antigens and 9 patients developed percipitins to cytoplasmic antigens during this period. The association between the formation of precipitins and the extent of faecal colonization is discussed.