SUMMARYPseudomonas aeruginosa strain C l o when exposed to phage Pc which had been inactivated by heat or acid produced a factor which decreased the yield of Pc phage in a single cycle of growth on P. aeruginosa C l o . Some of the factor appeared in the culture fluid, but most could be obtained only by disrupting the bacteria. The factor did not inactivate free Pc phage. It decreased phage yield most when it was mixed with P. aeruginosa ClO 1 hr. before infection with Pc phage and was then kept present to the end of the experiment. When the factor was diluted out immediately after adsorption of Pc phage on P. aeruginosa C 10, or when it was present only from the start of adsorption, it had less effect on phage yield.