28 patients with aerobic (7), anaerobic (7), and mixed (14) infections were treated with intravenous piperacillin for an average duration of 14 days. All bacterial isolates tested in this study were susceptible to piperacillin less than or equal to 128 micrograms/ml. 27 patients were treated with 20 g of piperacillin/day in four divided doses. 1 patient with renal failure received only 8 g/day. 26 patients were treated with piperacillin and two with Staphylococcus aureus infection received gentamicin in addition. The peak serum levels were 266 micrograms/ml at 1 h and the trough 16 micrograms/ml at 5 h. In all, 26 patients (93%) were 'cured', one had 'recurrence', and one 'failed'. Adverse effects were pruitus in two patients, transient elevation of LDH in one, and transient eosinophilia in another. In our preliminary study, piperacillin was found to be safe and effective in the treatment of clinical infections.