Therapeutic doses of colistin sulphomethate sodium B.P. (Colomycin), pento-N sulphomethylpolymyxin B sodium (Thiosporin) and ampicillin B.P. (Penbritin) as well as normal saline have been given to volunteers with normal kidney function.2. A blind crossover technique was used and the effects of the compounds on blood levels and clearances of urea and creatinine studied.3. All four treatments resulted in some changes in the parameters measured but were reversible. The most marked changes were found with pento-N sulphomethylpolymyxin B sodium and were also associated with severe side-effects.4. In contrast to penicillins it is established that the process of tubular secretion sensitive to probenecid block plays no part in the renal excretion of polymyxins.