Bacteriological filtrates of three strains of Pseudomonas aeruginosa were compared with respect to inhibitory effect on ciliary movements and a quantitative difference was established between them. The cilia inhibitory effect was strictly concentration-dependent and was resistant to heating. The ciliotoxic effect disappeared from filtrates after chloroform extraction. The chloroform-extracted sediment ws dissolved in physiological saline and the solution revealed an inhibitory effect on cilia. Involvement of endotoxin is not probable, since E. coli endotoxin in a high concentration was not toxic. Partially purified Pseudomonas aeruginosa phenazine pigment as well as haemolysin inhibited ciliary activity and the effect was standardized in the present experimental system.