In vitro studies of the uptake, metabolism, and release of tiazofurin by human red blood cells reveal extensive phosphorylation with intracellular trapping of drug predominantly as tiazofurin triphosphate. This phenomenon may explain, in part, the plasma pharmacokinetic profile of tiazofurin. Metabolism of tiazofurin to tiazofurin adenine dinucleotide, the presumed oncolytic metabolite, occurs in mononuclear blood cells but not in erythrocytes.