EXPOSURE to ionizing or ultra-violet radiation or radio-mimetic chemicals has long been known to induce the vegetative reproduction of certain temperate bacterio-phages in lysogenic bacteria¹. Cytotoxic agents, such as mitomycin C (ref. 2), and folic acid antagonists, such as aminopterin³, have also been reported to be inducing agents. Recently, phage induction has been observed after thymine deprivation in lysogenic thymine-deficient mutant strains^4,5.