The number of haemolytic plaque forming cells (PFCs) in the spleens of normal mice could not be increased by the injection of nucleic acids. However, when nucleic acids were injected into mice simultaneously with a priming dose of 3 x 10⁶ sheep erythrocytes (SRBCs) an appreciable stimulatory effect was observed. The same dose of SRBCs did not result in an increase in PFCs when injected alone.
Nucleic acids at a concentration of approximately 1.5 mg per mouse resulted in an optimal stimulation of the immune response. Treatment of the nucleic acids by ribonuclease and deoxyribonuclase essentially eliminated the stimulatory effect of the RNA and DNA respectively whereas incubation with trypsin did not. Administration of pure single stranded RNAs from the livers and spleens of both normal and immunized mice resulted in a greater increase in the number of PFCs than nucleic acids from other sources. The stimulation of PFCs by RNAs from immunized mice was slightly greater than by RNAs from non-immunized mice. The immune response was also activated if the nucleic acids and the priming dose of antigen were not injected simultaneously.