Human leukocytes and platelets were labeled in plasma with indium 111, by incubating cells first with 2-mercaptopyridine N-oxide (Merc) and then with ionic or weakly chelated 111In citrate. We investigated the mechanism by which this procedure enabled us to label cells in plasma. The interactions of 111In and Merc with cell membrane and cytoplasm were studied by homogenizing labeled cells and analyzing the homogenate by gel filtration. Studies were facilitated by use of sulfur 35-labeled Merc. Although 65% +/- 12.8% of added 111-In was incorporated in cells in presence of extracellular Merc, only 0.8% +/- 0.1% and 5.1% +/- 3.2% 35S-Merc was associated with 10(8) leukocytes and 10(10) platelets, respectively. In the absence of extracellular Merc, only 4.5% +/- 0.3% 111In was taken up by the cells. In each type of cell 83% to 99% of the cell-incorporated 35S Merc was associated with a cytoplasmic component with apparent molecular weight 5200 daltons, independently of the presence or absence of radioactive or stable indium. An approximately equal proportion of 111In was bound to similar cytoplasmic components in both types of cells. On adenosine diphosphate-induced platelet aggregation, less than 3% of platelet-bound 111In or 35S-Merc was released. These results indicate that it is the extracellular Merc that facilitates 111In labeling. It does not bind to cell membrane, but forms a lipid-soluble complex with 111In. This complex passively diffuses through the cell membrane, allows 111In to bind to cytoplasmic components, and provides a stable cell label.