The binding of 2,3-dimercapto-1-propanesulfonate (DMPS) in plasma was determined in three healthy young adults after a single 300-mg p.o. dose. By 5 hr after DMPS administration, 62.5% of the total plasma DMPS was bound to proteins. The remainder consisted of nonprotein associated DMPS disulfides (36.6%) and unaltered DMPS (0.9%). Protein-bound DMPS consisted of a DMPS-albumin complex (84%) and a higher molecular weight protein complex (16%), perhaps albumin aggregates. DMPS was released from the isolated DMPS-albumin complex after treatment with dithiothreitol, indicating that it was bound via a disulfide linkage. The half-life of unaltered DMPS was 1.8 hr, whereas that of altered DMPS was 20 hr, suggesting that the DMPS-albumin disulfide complex is stable and that DMPS was released from it slowly. In addition, the biotransformation of OMPS to disulfide forms was extensive. By 9 hr after administration, 10% of the total urinary DMPS was unchanged drug and 90% was altered DMPS. The latter was converted to DMPS by dithiothreitol, indicating that the altered DMPS consisted of disulfides. In 2- to 4-hr urine, DMPS disulfides included cyclic polymeric DMPS disulfides (97%), DMPS-cysteine (1:2) mixed disulfide (2.5%) and acyclic DMPS disulfide (0.5%). The cyclic polymeric DMPS disulfides were present in a major (91.5%) and minor (5.5%) form. DMPS-albumin mixed disulfide and nonprotein DMPS disulfides may prolong the heavy metal mobilizing activity of DMPS and thus may represent reservoirs of DMPS which can be released by disulfide reduction in vivo.