SUMMARYAdministration of splenic microsomal RNA extracts to pyridoxine-deficient rats had been previously shown to increase viability of subsequent skin homografts. The following observations on the specificity of action of splenic extracts have been made: (1) treatment of splenic microsomal RNA extracts with ribonuclease or alkali destroyed their ability to prolong viability of skin grafts; (2) splenic microsomal RNA extracts administered in combination with splenic extracts prepared from the mitochondrial or soluble fractions had no beneficial effect; (3) an RNA extract obtained from a rat of genetic strain other than that of the skin donor was ineffective; and (4) microsomal RNA derived from the spleen of the skin donor failed to increase viability of the skin graft when administered to a normal recipient.