The LD 50/30 days for 200 kV X-rays, total body irradiation in Swiss mice was 575 r/air. The corresponding LD50 for different neutron sources varied between 210 and 490 rep. Thus a relative biological efficiency (RBE) varying between ? and 1.2 for neutrons was established. With respect to cataract formation the RBE for neutrons varied between 5.0 and 2.5. With X rays the dose which induced cataracts in 50% of the exposed animals (CD50) was 1.35 times larger than the LD50. However, irrespective of the neutron source, the CD50 for neutrons was less than the LD50, (approximately 0.6 to 0.8 of the neutron LD50).