[en] Californium-252 is potentially of use in medicine as a replacement for radium in interstitial and intracavitary applications. The expected advantage is based on the lower oxygen enhancement ratio (OER) characteristic of the mixture of neutrons and gamma rays emitted by ²⁵²Cf, compared with the pure gamma-ray emission from radium. OER values have been measured over a wide range of dose-rates from 10 rads per hour up to several hundred rads per hour. At the lower dose-rates, the OER of the mixed neutron and gamma-ray emission from ²⁵²Cf is found to be approximately 1.5, which is indistinguishable from that characteristic of fission spectrum neutrons alone. This is explained by the fact that, at these exceedingly low dose-rates, the gamma-ray component has a negligible biological effect. As dose-rate is increased to approximately 100 rads per hour, the biological effectiveness of the gamma rays can no longer be ignored, and the OER that is obtained is a compromise between the 1.5 characteristic of fission spectrum neutrons, and the 2.5 to 3 characteristic of gamma rays. This increase of OER with dose-rate has some bearing on the range of applications of ²⁵²Cf in radiotherapy. The OER for this mixture of neutrons and gamma rays emitted by ²⁵²Cf can be calculated and predicted from values for neutrons and gamma rays alone. (author)