The relative biological effectiveness (RBE) for liver tumor induction by fission fragments was evaluated. Data were taken from a previous study in which beagle-dogs were injected with up to 10.7 kilobecquerels per kilogram californium-249 (15237975) (Cf249) or californium-252 (13981174) (Cf252) and maintained for lifespan observation. Data on the incidence of malignant and benign liver tumors obtained at necropsy were analyzed. Risk coefficients for liver tumor induction by fission fragments were calculated by normalizing the number of observed liver tumors minus the incidence of tumors in the controls at the total dose received by the dogs at risk to 10(4) dog-gray (dog-Gy). The RBE for induction of liver tumors by fission fragments was calculated by an equation that incorporated the risk coefficients for Cf249 and Cf252 and a factor that partitioned the Cf252 dose into fractions derived from alpha particles and fission fragments. For malignant liver tumors, the risk coefficients were found to be 190 per 10(4)dog-Gy for dogs given Cf249 and 280 per 10(4)dog-Gy for dogs given Cf252. The RBE for induction of malignant liver tumors by fission fragments was 2+/- 3. For all liver tumors, the RBE was 7+/-14. The authors conclude that the RBE for induction of liver tumors by fission fragments, versus alpha particles, may be greater than 1.0. The actual value of the RBE may vary according to the deposition pattern of the emitters, dose, and the biological endpoint being considered.