SUMMARY: Two new compounds inhibited the increase of phage when two phages of Pseudomonas pyocyanea were grown on an indicator strain. These compounds also inhibited the appearance of iridescence in a lysogenic strain of this organism.One iridescent lysogenic strain showed a rise in phage titre a t 1&2 hr., which corresponded to the latent period of the phage when it acted on an indicator strain.Both these increases were inhibited by the compounds. This lysogenic strain also showed a rise in phage titre a t 4 hr. but this was unaffected by the compounds. The final phage titre of the lysogenic culture was not affected significantly, unlike the complete and permanent suppression of phage increase when free phage acted on an indicator strain. Cultures and filtrates of three lysogenic strains of Ps. pyocyanea showed a great increase in phage titre when assay plates (against an indicator strain) were incubated for a further period after the initial reading a t 18 hr.Many workers have described iridescent strains of Pseudomonas pyocyaneu. Warner (1950) gave a. detailed description and concluded that the iridescent clearing was not caused by bacteriophage action but was a distinct lytic phenomenon, probably the result of bacterial variation. Don & van den Ende (1950) studied thirty strains of Ps. pyocyama with particular reference to the metallic lytic effect and bacteriophage action but the relationship, if any, between these two effects remained obscure. Following an investigation of drug action on the phages of Ps. pyocyanea (Dickinson, 1948), an explanation was sought for the lack of action of drugs on phages associated with iridescent cultures of this organism. It was decided to investigate the reproduction of phage in one of these cultures. The present paper reports this work and includes observations which correlate iridescence with bacteriophage action in this particular strain.The following abbreviations and definitions are used in this paper.