The administration of [14C]putrescine or [14C]spermidine (i.v., 100 muCi) to normal volunteers or patients with advanced cancer resulted in alpha-phase half-lives of 40 and 30 sec and beta-phase half-lives of 30 and 60 min, respectively. No significant differences were found between the plasma decay curves of normals and those of cancer patients. Urinary excretion was similar with both groups excreting approximately 45% of [ 14C]putrescine within 24 hr and 60 to 76% of [14C]spermidine within 48 hr. Dowex chromatography indicated that more than 90% of the radiolabel in the urine was in a conjugated form. Plasma conjugation studies of [14C]putrescine and [14C]spermidine in both groups indicated near-total conjugation of the radiolabel within 4 to 5 min of i.v. injection. Since putrescine and spermidine are markers of disease activity, characterization of the conjugates will be important for the development of rapid, specific tests of altered disease activity in response to multimodality therapy.