Proflavine in concentrations which had only small effects on the growth rate of the host organism markedly decreased the phage yield from bacteria infected with phage λ gv. The number of bacteria which released infective phage particles was reduced and the bacteria which remained productive released a smaller number of particles/bacterium. One of the inhibitory effects of proflavine became apparent about the time when phage assembly began. The length of the latent period was not affected. In addition to its effect on the production of complete phage particles, proflavine inhibited the synthesis of infective λ DNA. The former process was the more sensitive to proflavine inhibition. Exposure to proflavine immediately after infection with phage λ+ caused vegetative phage reproduction in many bacteria which would otherwise have become lysogenic for λ+.