The labelling index of the lymphoid cell population in the small intestinal epithelium of the mouse was determined at various time intervals after single or multiple intraperitoneal injections of tritiated thymidine. The results show that the population is composed on both short-lived and long-lived lymphoid cells. The generation time of short-lived cells was estimated at 10–14 h. Although the long-lived lymphocytes appear to have a generation time in the order of months, the actual number of these cells present in the epithelium declines fairly rapidly with a half-life of approximately 7 days.