Pseudomonas aeruginosa pneumonia was produced in dogs with radiation- induced leukopenia to study the comparative efficacy of several different therapies. In a randomized control trial, five treatment regimens were compared: no antibiotics or granulocytes (controls), gentamicin (5 mg/kg/day), carbenicillin (500 mg/kg/day), gentamicin and carbenicillin (same dosages), and daily granulocyte transfusion (minimum 5 x 10(9) cells/day) plus gentamicin (5 mg/kg/day). The most effective therapy was gentamicin plus granulocyte transfusions. Gentamicin alone was not significantly better than no specific therapy. Carbenicillin with or without gentamicin gave intermediate results. This study further supports the utility of granulocyte replacement therapy of infections in severely granulocytopenic subjects. The results also indicate that the relative value of granulocyte transfusions depends upon the specific antibiotic regimen with which these transfusions are compared.