Leukocyte kinetics were investigated in 22 patients undergoing cardiopulmonary bypass to determine the nature of the changes in the white blood cell count associated with this procedure. Both polymorphonuclear leukocytes and lymphocytes were taken up by the lung as pulmonary blood flow was lowered, but only polymorphonuclear leukocytes were taken up as blood flow was restored. The complement 3a level was increased approximately twice the control value within 2 minutes of going on bypass and remained elevated throughout the procedure. The peripheral white blood cell count doubled during the bypass procedure owing to a release of polymorphonuclear leukocytes and their precursors from the bone marrow. The increase in polymorphonuclear leukocytes was prevented by hyperthermia but reappeared quickly when the body temperature was restored to 36 degrees to 37 degrees C.