Natural resistance in mice to Pseudomonas aeruginosa was decreased 10 fold with a single dose of 300 mg of cyclophosphamide (CY) per kg intraperitoneally. Mice were resistant to infection when immunized actively with Pseudomonas vaccine or passively with Pseudomonas immune serum before receiving CY. Syngeneic spleen, thymus and/or bone marrow cells were transfused into CY treated recipient mice. Protective anti Pseudomonas antibody was elicited in the recipient mice when they were vaccinated 1 day after receiving normal spleen cells and challenged 8 days after vaccination. When 1.6 x 107 normal thymus and bone marrow cells were infused before vaccination, 69% of the recipients of both cell preparations responded serologically compared with 15 and 27% of those receiving either thymus or bone marrow cells, respectively. CY treated thymus or bone marrow cell recipients were resistant to Pseudomonas infection when 6 x 107 of either cell population was transfused.