BALB/c mice immunized by intraperitoneal injection of purified Pseudomonas aeruginosa lectin preparations are fully protected against a lethal dose of the live bacteria. Intraperitoneal inoculation of spleno-cytes or bone marrow cells obtained from actively immunized mice into naive syngeneic mice was shown to significantly increase their resistance to P. aeruginosa infection. A similar transfer of splenocytes or bone marrow cells from untreated control mice did not provide protection against a lethal Pseudomonas challenge. Administration of immunocompetent cells from immunized animals to naive mice by the i.v. route was less efficient than the i.p. inoculation, probably due to different homing of the cells. Only the i.p. injection concentrated the immune cells to the site of infection.