RNA polymerase from Micrococcus lysodeikticus has been used to synthesize rG : rC† from rC and GTP. The rG: rC product appears to consist of one continuous rC strand and many short pieces of rG comprising the other strand. rG:rC is completely resistant to pancreatic RNase at 37°C. Attempts at thermal dissociation of the complex were unsuccessful. Dissociation can be achieved in alkali, for which the p K a is about 12·5 at 25°C. Alkaline dissociation is essentially reversible, but the complex formed by neutralization is distinguishable from the rG:rC synthesized initially. The density of rG:rC in cesium sulfate is 1·685.