IT has been the general experience of phage investigators1 that bacterial viruses reach their highest titres when preying on rapidly growing cultures, differing thereby from the large group of plant and animal viruses, which are able to multiply in host cells regardless of stage of development. Krueger et al.2 showed that staphylococci will support phage reproduction in the presence of penicillin which inhibits their own growth; but such organisms must first be ‘activated’, which means they must have entered the logarithmic phase of growth.