I.C.R.F. 159, a new antitumour agent, has been assessed in six patients with acute leukaemia and three with lymphosarcoma. In all but two there was a considerable fall in circulating primitive cells, and in one there was bone-marrow evidence of a partial remission. Severe toxic effects were seen in only one case; they consisted of alopecia and gastroenteritis. It is suggested that I.C.R.F. 159 is worth further examination in all forms of acute leukaemia and lymphosarcoma.