Experimental intraabdominal abscesses were produced in mice by intraperitoneal injection of Bacteroides fragilis and Pseudomonas aeruginosa. The therapeutic efficacy of rifampicin and cefsulodin alone, and in combination was investigated in this in-vivo experimental mixed intraabdominal abscess model. Treatment with rifampicin at 10, and 25 mg/kg or cefsulodin at 50, and 100 mg/kg singly or in combinations prevented mortality as compared to 68% mortality rate occurring in the untreated mice. Rifampicin, at 25 mg/kg dose, was very effective in preventing abscess formation and produced bacterial eradication. It prevented abscess formation in 80% of the mice and eradicated both Bacteroides and Pseudomonas in 100% and 75% of the abscesses of the mice. Cefsulodin failed to reduce the incidence of abscess formation, and to eradicate Bact. fragilis from the abscesses, although it significantly decreased Ps. aeruginosa in the abscesses. The combination of rifampicin at 10 mg/kg and cefsulodin at 100 mg/kg was more effective than either of the antibiotics alone and was as effective as rifampicin alone at 25 mg/kg levels. This combination was bactericidal against both organisms in the infected mice.