The properties of an RNA-A protein complex isolated from the RNA bacteriophage M12 are described. The molar ratio of RNA to A protein in the complex is estimated to be 1:1. In sucrose gradients, the complex sediments like free RNA molecules. In contrast to RNA alone, which can only infect spheroplasts, the RNA-A protein complex infects intact Escherichia coli cells and produces infectious progeny particles like the original phage. Evidence is presented that the infection of the host cells by the complex takes place via F pili. All of the infectivity disappears if the ionic bonds of RNA to A protein in the complex are dissociated in 0.5 M sodium chloride buffer at 37 degrees C. Furthermore, the kinetics of complex dissociation and loss of infectivity are the same, implying that the binding of A protein to the RNA is a prerequisite for infectivity on intact host cells.