The biological effects of radionuclides that emit alpha particles are of considerable interest in view of their potential for therapy and their presence in the environment. The present work is a continuation of our ongoing effort to study the radiotoxicity of alpha-particle emitters in vivo using the survival of murine testicular sperm heads as the biological end point. Specifically, the relative biological effectiveness (RBE) of very low-energy alpha particles (3.2 MeV) emitted by 148Gd is investigated and determined to be 7.4 +/- 2.4 when compared to the effects of acute external 120 kVp X rays. This datum, in conjunction with our earlier results for 210Po and 212Pb in equilibrium with its daughters, is used to revise and extend the range of validity of our previous RBE-energy relationship for alpha particles emitted by tissue-incorporated radionuclides. The new empirical relationship is given by RBE alpha = 9.14 - 0.510 E alpha where 3 < E alpha < 9 MeV. The validity of this empirical relationship is tested by determining the RBE of the prolific alpha-particle emitter 223Ra (in equilibrium with its daughters) experimentally in the same biological model and comparing the value obtained experimentally with the predicted value. The resulting RBE values are 5.4 +/- 0.9 and 5.6, respectively. This close agreement strongly supports the adequacy of the empirical RBE-E alpha relationship to predict the biological effects of alpha-particle emitters in vivo.