Five antimicrobial drugs (ceftazidime, ciprofloxacin, imipenem, piperacillin and tobramycin) and two human, intravenously applicable, IgG immunoglobulin preparations (Psomaglobin, Polyglobin N) were examined alone and in various combinations for therapeutic efficacy in myelosuppressed as well as normal NMRI mice following systemic infection with 4 selected strains of Pseudomonas aeruginosa. Both IgG preparations alone invariably failed to passively protect neutropenic and normal mice, even though the preparations contained demonstrable antibodies against purified exotoxin A of P. aeruginosa and against numerous polypeptide and lipopolysaccharide constituents of the 4 test strains of P. aeruginosa. Among the antimicrobial drugs employed alone, ciprofloxacin and imipenem were the most effective. Tobramycin alone occupied an intermediate position, whereas monotherapy with ceftazidime and piperacillin invariably failed. All antimicrobial drug combinations were efficacious in myelo-suppressed as well as normal mice. Both IgG preparations failed to enhance the activities of ceftazidime and piperacillin in neutropenic mice; the activity of tobramycin was augmented against 1 of 2 P. aeruginosa strains, as well as that of ciprofloxacin. Both IgG preparations enhanced the activities of ceftazidime and tobramycin in normal NMRI mice, as well as that of piperacillin against 1 of 2 test strains. It was concluded that the two IgG preparations were more beneficial for normal rather than neutropenic mice, provided that the animals were treated with appropriate antimicrobial drugs as well.