Twenty-five consecutive patients admitted to hospital with severe soft tissue infection were entered into an open trial designed to evaluate safety and efficacy of the drug combination tazobactam/piperacillin. The dosage regimen was 4 g piperacillin/500 mg tazobactam Q8H. There were twenty cases of uncomplicated cellulitis and five cases with associated abscess formation. These five cases required adjunctive surgical drainage. Mean duration of therapy was 7.6 days. No other antibiotics were administered unless treatment with the study drug failed. There were eight treatment failures, six related to the trial drug. Four patients developed an allergic response to the trial drug, necessitating a change of therapy. Three patients failed to respond; all three had acute cellulitis in association with peripheral vascular disease. Significant bacterial isolates were grown in thirteen patients; Group A streptococci in three, S. aureus in five, other pathogenic streptococci in four and coliforms or Ps. aeruginosa in five. The majority of isolates except the streptococci were resistant to piperacillin but all isolates except one strain of Ps. aeruginosa were susceptible to the combination. The combination is suitable for the treatment of serious soft tissue infection, but increased doses may be appropriate in infection at poorly perfused sites.