Calprotectin is an abundant cytosolic protein complex of humanneutrophils with in vitro extracellular antimicrobial activity. Studiessuggest that calprotectin may be actively secreted from intact HL-60cells and that it can be translocated to polymorphonuclear neutrophil(PMN) cell membranes. To examine whether calprotectin is secretedextracellularly, we incubated soluble and particulate stimuli,including live and heat-inactivated Candida albicans, withwhole blood and measured calprotectin levels in the plasma. We comparedthe release of calprotectin to that of lactoferrin, a protein known tobe secreted by PMNs. Extracellular lactoferrin was detected afterincubation with any of the particulate stimuli. In contrast, asignificant increase in extracellular calprotectin was found only afterincubation with live C. albicans. Specifically, theincrease in extracellular calprotectin correlated directly with aproportional decrease in PMN viability. Our results indicate that humanPMN calprotectin is not secreted extracellularly except as a result ofcell disruption or death.