1. Contribution from the Gray Laboratory Cancer Research Trust, Mount Vernon Hospital, P.O. Box 100, Northwood, Middlesex HA6 2JR, U.K., Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai-400085, India, and MRC Radiobiology Unit, Chilton, Didcot, Oxon OX11 ORD, U.K.