There is good biological evidence for an enhanced effect of californium 252 on tumors because of a reduced necessity for the presence of oxygen during the period of irradiation. This is the primary reason for consideration of its use in interstitial and intracavitary cancer therapy. Balanced against this is the complexity of dosage calculation and the variability of RBE depending on dose rate. The difficulties in storage and handling due to special hazards must also be considered. Because of these problems a careful evaluation of californium 252 must be carried out prior to its routine clinical use. With the known difficulties in carrying out such evaluations (lack of sufficient suitable clinical material in any one institution, variability in clinical problems with the same malignancy, differences in technique of application and reluctance to use a new modality in situations where standard methods are relatively successful) it would seem wise to have a collaborative clinical trial involving a number of cooperative institutions under supervision on a national level.