Indium-111 leukocyte scanning has evolved into a practical and highly accurate method for the identification of infectious and inflammatory processes. The most commonly used agent for labeling leukocytes has been [111In]oxine. We have investigated a newer agent, 2-mercaptopyridine-N-oxide (Merc) at our institution which unlike oxine, allows us to label leukocytes in plasma, using a simple kit procedure. Of the 92 consecutive patients referred for detection or localization of an infectious process, autologous leukocytes of 55 patients were labeled with [111In]Merc, while those of the remaining 37 patients were labeled with [111In]oxine. The sensitivities for Merc and oxine procedures were 87% and 92%, respectively, while the respective specificities were 100% and 92%. We conclude that the [111In]Merc-labeled leukocytes are equally effective as [111In]oxine-labeled leukocytes in detecting infectious processes. The use of [111In]Merc is advantageous over [111In]oxine for white blood cell labeling because of its easier preparation.