Bioptic material of the small intestine is suitable for short-time in-vitro incubation in 3H-thymidine labelled medium followed by autoradiography. Histological and autoradiographic investigations of the small intestine of 45 children yielded the following characteristic findings: 1. 93 per cent of the healthy controls (n = 14) showed normal mucous membrane (Type I according to Shmerling) and a mean 3H-thymidine labelling index of 13 per cent. 2. Normal histological findings but a mean autoradiographic labelling index of 22 per cent were recorded from 87 per cent of children with cow's milk or fructose incompatibility (n = 12). 3. Mucous membrane of Shmerling Type I and a labelling index of enterocytes of 32 per cent was recorded from 50 per cent of children with coeliac disease (n = 19) after diet periods of seven or three to five months. Other children with coeliac disease but without therapy up to that time showed in 80 per cent the Shmerlings Type III and in 20 per cent Type II accompanied by the highest labelling index of 35 per cent. The increase of proliferative activity of enterocytes in coeliac disease shown by autoradiography and histological observations (villous atrophy) suggested an increased rate of cell death which led to the need for high cell replacement. This autoradiographic method used complementarily to usual histological diagnosis, has proved to help saving an enormous amount of time in the diagnosis of coeliac disease.