The potential benefit of the introduction of californium-252 in interstitial and intracavitary therapy is related to the greater efficiency of its neutron emission against anoxic cancer cells. In that respect, the oxygen enhancement ratio (OER) of the 252Cf emission has been determined for a continuous low dose rate irradiation. The biological system is growth inhibition in Vicia faba bean roots. A new Vicia faba "BelB" strain has been used, which better tolerates long periods (up to about 10 hours) of anoxia. In a first series of experiments, for a 252Cf (Dn + gamma) dose rate of 0.11 Gy . h-1, an OER of 1.4 +/- 0.1 was observed (the gamma contribution of D gamma to the total absorbed dose Dn + gamma was 0.35 at the position of the root tips). In a second series of experiments, in somewhat different geometrical conditions with a 252Cf (Dn + gamma) dose rate of 0.13 Gy . h-1, an OER of 1.5 +/- 0.1 was observed (D gamma/Dn + gamma = 0.42). The OER values observed for similar irradiation times, with iridium-192 gamma-rays, were 2.3 +/- 0.2 and 2.6 +/- 0.1 respectively, which leads to therapeutic gain factors (GF) of 1.6 and 1.7 respectively. These GF values are slightly lower than those previously obtained (GF = 1.8) on the same system, with d(50)-Be, p(75)-Be and 15 MeV neutron beams.