- Normal oesophageal mucosa obtained during upper abdominal surgery or urothelium obtained from kidney transplants was placed in explant culture and exposed to 60Co gamma radiation after 48 h. Cultures were maintained for two to six weeks after exposure and were monitored at various intervals for the development of features associated with malignant transformation. Endpoints examined included proliferation rate, frequency of proliferating cells, cell type distribution and degree of differentiation of the different cell types. The results indicate that following exposure to gamma rays (2.5-10 Gy) an increased overall growth rate of the surviving cells can be observed 2-4 weeks later. Analysis of the results using autoradiography confirms that a higher level of cell proliferation occurs in treated cultures than in the control untreated cultures. When the distribution of different cell types in the culture is examined, the increase in growth can be seen to be due to greatly increased numbers of endothelial cells. These proliferated over the surface of the epithelial cells and are more strongly positive for endothelial cells markers than endothelial cells occurring in control cultures. The degree of differentiation of endothelial cells into capillary like structures is also more apparent in carcinogen treated cultures. Foci expressing both epithelial and endothelial markers also occur. The results suggest that exposure of tissue fragments to radiation stimulates the growth and development of endothelial cells in resulting cell cultures. The effect may be due to a direct action of the treatment on the endothelial cells but it is more likely that it results from a secondary effect mediated by traumatic response of damaged epithelial cells.