The protective effect of pre-irradiation injections of diethylaminoreserpine (DL-152) for normal and malignant tissues in the mouse has been investigated. Dose modifying factors (DMFs) obtained for normal tissue ranged from 1.0 for bone marrow CFUs to more than 1.8 for skin. The DMFs for two transplantable tumours investigated were 1.0 for the EMT6 adenocarcinoma and 1.70 for the KHT fibrosarcoma (at a surviving fraction of 0.1. Acutely hypoxic KHT tumours were protected to a slightly lesser extent than were aerated tumours. For the KHT tumour, the number of clonogenic cells recovered from non-irradiated tumours one hour after DL-152 injection was reduced to 60 per cent of the number covered fro saline-injected controls, while, if DL-152 injected mice were acutely hypoxic at the time of sacrifice, the number of clonogenic cells was further reduced. The survival of non-irradiated EMT6 tumour cells was unaffected by DL-152 injection prior to sacrifice.