When it was first observed that the strain 15 of Escherichia coli, after u.v.-irradiation and further incubation, would eventually lyse and release an anti-bacterial agent, this agent was considered to be a ‘colicine’ (Mukai, 1960; Ryan, Fried & Mukai, 1955). The possibility that, in addition, a defective phage was produced, was left open. But, since in an u.v.-induced lysate of this strain all...
