The nitroxyl radical 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (Tempol) is reported to elicite some effects on different biological models. This paper studied the influence of Tempol on the therapeutic action of an alkylating agent cyclophosphamide (CP) in the transplantable murine lymphosarcoma LS. When administered exactly before CP, Tempol exerted no influence on efficacy of the tumour therapy but significantly reduced it under a single or multiple preliminary injections 1-3 days before CP. This regimen of Tempol administration is found to elevate the activity of aldehyde dehydrogenase in the liver, the enzyme which is known to reduce the yield of the activated metabolite(s) of CP.