The radiation sensitivity of five human lymphoma cell lines has been investigated. All cell lines were originated from tumours clinically classified as histiocytic lymphomas. Using biochemical and immunologic techniques it was found that only one of them was a true histiocytic lymphoma. This cell line had an appreciable shoulder on the survival curve and capacity to repair sublethal damage. The survival curves of the other cell lines had no shoulders and the cells had nearly no capacity to repair sublethal damage. Taken together with previously published experimental and clinical observations the results indicate that tumours which by conventional diagnostic techniques are classified as histiocytic lymphomas are very heterogeneous in radiation sensitivity. It is possible that tumours of true histiocytic origin are more resistant to radiation than tumours of lymphocytic origin.