Abstract
The morphology and physico-chemical properties of a temperate phage, B33, of Pseudomonas aeruginosa have been determined. This phage is similar in size and structure to the previously described and serologically related phage B3 (Holloway, Egan & Monk, 1960), but differs from it in its plating properties on bacteria harbouring R plasmids. The plasmid RP1-1 causes a reduction in e.o.p. of B33 of 10(- 6), and the mechanism whereby this occurs has been studied. The interaction is a specfic one since other plasmids either fail to affect plating or completely abolish it. The mechanism by which the latter occurs is different from that mediated by RP1-1, since mutants of B33 insensitive to RP1-1 nevertheless fail to plaque on these hosts.