Antibacterial activity of tobramycin in combination with carbenicillin or cephalosporins against 20 strains of Providencia stuartii was studied. The combinations of tobramycin with carbenicillin (1 : 2.5) or cephaloridine (1 : 1) were most active. The MIC of tobramycin used in combination with carbenicillin or cephaloridine decreased 3-30 times. In treatment of albino mice with sepsis caused by Providencia stuartii it was possible to lower 2-8 times the dose of tobramycin used in combination with carbenicillin or cefazolin.