Transfectivity titers of RNA preparations obtained from purified poliovirions in phosphate-buffered saline using phenol were low. Addition of tissue culture nutrient medium to the virions prior to extraction with phenol increased the RNA titers 100–1000-fold. The 32 solute differences between the phosphate-buffered saline and the nutrient medium were divided into three blocs for testing. Only the bloc containing the 13 amino acids of the nutrient medium enabled the preparation of high-titer RNA. Tests of the individuals amino acids revealed that l-histidine has high activity, l-cystine and l-glutamine moderate activity, and the remaining ten zero or very slight activity. Five congeners of l-histidine, viz. d-histidine, histamine, l-histidine methyl ester, α-N- acetyl- l-histidine , and l-histidyl-glycine, also had high activity; but imidazole had no activity. The histidine effect was obtained whether transfection was enhanced by DEAE-dextran or by bentonite. Histidine was fully effective only when it was added to the virions before or very shortly after the phenol; later additions of histidine were progressively less effective. Without added histidine, RNA preparations made very rapidly and inoculated promptly showed high transfectivity titers, but the transfectivity was highly labile; with histidine present, the high RNA titers were stable. Histidine did not reactivate the inactivated RNA.