Active protection against Pseudomonas aeruginosa could be induced in mice by immunization with either the phenol killed cells or the alcohol precipitated fraction of the slime layer, or the ribosomal vaccine preparation. Passive protection could also be induced by injecting into mice antisera prepared in rabbits against these bacteria. This protection was due to the production of antibodies in reaction to the slime layer; the absorption of these antibodies by the slime caused the loss of protection. The fact that mice were also protected by vaccination with strains other than those used for challenging was attributed to the presence of an antigenically similar slime. Passive protection towards a heterologous strain, even one with an antigenically similar slime layer, was dependent on the dose of the challenging injection.