SummaryThe intestine of the foetal lamb was exposed to large quantities of δ‐globulin (IgG) by prolonged intra‐duodenal infusion, and absorption of intact IgG, with transfer to the lymph, continued undiminished, i.e., there was no evidence of closure. The rate of proliferation of the intestinal epithelium of the foetal and newborn lamb was measured using mitotic indices and localised labelling with (3H)‐thymidine (TdR). In the foetus, cell division in the crypts occurs at a lower rate than the newborn (p<0·001) and there is very slow replacement of the intestinal epithelium. In the newborn lamb, a portion of the small intestine was incubatedin vivowith TdR and the progress of labelled cells from the crypts upwards along the villi estimated, using autoradiography of serial biopsies from the same animal. A front of mature, digestive epithelium could be seen advancing up the villi, displacing the immature foetal type of cell which was capable of transfer of intact IgG to the lymphatics of the intestine. The evidence presented supports the hypothesis that immediately after birth the intestinal epithelium of the lamb begins to be replaced by a digestive type of cell, and the layer of cells responsible for absorption of colostral antibodies progressively disappears from the villi, resulting in closure.