Polyamine biosynthesis is important for cell proliferation and growth. The purpose of this study was to determine the biochemical and pharmacological parameters associated with host toxicity from a continuous infusion of alpha-difluoromethylornithine (DFMO). Twenty-five patients with metastatic carcinoma of the colon or rectum received continuous infusion of DFMO at a median dose of 8 g/m2/day (range, 6-14) for 28 days. DFMO plasma levels, RBC, plasma putrescine, spermidine, and spermine levels, and patient toxicities were evaluated. There was a significant decrease in RBC and plasma levels of putrescine, spermidine, and spermine following DFMO administration compared with the baseline RBC and plasma levels. Pearson correlation coefficient comparing nadir platelet count and steady-state DFMO level was statistically significant (n = 37; P less than 0.01; r = -0.53). Sustained suppression of circulating polyamine levels was also achieved with continuous DFMO infusion. The correlation between steady-state plasma DFMO levels and lowering of platelet count warrants prospective evaluation to determine its clinical usefulness.