Renal toxicity was assessed in 19 patients receiving methyl acetylenic putrescine (MAP), an irreversible inhibitor of ornithine decarboxylase. Patients received 250 mg t.d.s. for up to 13 weeks. This dose effectively inhibited the target enzyme, as shown by elevations in decarboxylated S-adenosyl methionine levels. No significant nephrotoxicity was observed in these patients as determined by plasma urea, creatinine and creatinine clearance measurements, although minor elevations of the urinary enzymes lactate dehydrogenase, N-acetyl-beta-glucosaminidase, alkaline phosphatase and alanine aminopeptidase were observed. As this could represent sub-clinical renal damage, caution should be exercised when using MAP in combination with other cytotoxic drugs.