Biopsy specimens of human colonic mucosa taken from the rectosigmoid of 12 normal subjects were maintained in explant culture for 4 days. Histological, microautoradiographic and chemical measurements were carried out to evaluate cell replication, the effect of deoxycholic acid, and the incorporation of uridine and leucine into RNA and protein. Active cell replication was shown to be greatest during the first day of organ culture, and the number of cells that synthesized DNA also increased when deoxycholic acid was added to the culture medium. At later times, with morphological evidence of tissue viability, the synthesis of DNA, RNA, and protein within colonic epithelial cells decreased, and the total number of cells in the crypt columns declined. Findings indicate good maintenance of metabolic activities of colonic epithelial cells in short-term explant culture, and the utility of both cell-kinetic and morphological observations in assessing the status of colonic explants at early and late intervals.