The radioprotective activity of eight selected copper (II) carboxylates--Cu (RCOO)2.nL (R = alkyl, aryl, 2-furyl and 2-thienyl; L usually represents water)--was assayed in a model of lethally gamma-irradiated (9 Gy, 0.97 Gy/min) mice. The compounds tested were applied (as solutions in saline) s.c. in three single doses of 20 mumol/kg 48.24 and 6 h before irradiation. The highest radioprotective effects were measured by survival of mice achieved after premedication of animals with copper (II) 2-thenoate monohydrate (77%), copper (II) acetylsalicylate (64%), copper (II) 2-methoxybenzoate monohydrate (62%) and copper (II) acetate monohydrate (54%). On the other hand, survival of vehicle-pretreated mice was only 10%. The observed biological properties of complexes are discussed in relation to their structures.