Previous investigators have proposed that gelatinase, a metalloproteinase found in neutrophils, is stored in a novel secretory compartment distinct from the two major granule populations, azurophilic and specific. To locate this proteinase in human neutrophils we reacted the cells for peroxidase and then applied monospecific polyclonal antibodies to human neutrophil gelatinase to immunolabel ultrathin frozen sections using an immunogold technique. Gelatinase was localized in a population of peroxidase-negative granules. Double-labeling experiments using antibodies against lactoferrin, a marker for specific granules, and gelatinase demonstrated colocalization of the two antigens in 80% of the specific granules. However, some granules immunostained with only the lactoferrin or gelatinase antibody. Similar techniques were used to examine precursor cells from bone marrow. In myelocytes both gelatinase and lactoferrin were present in large developing specific granules; however, some mature specific granules contained only lactoferrin. Thus, it is possible that lactoferrin synthesis begins earlier than gelatinase synthesis and that overlapping synthesis and segregation occurs during the myelocyte stage. These findings suggest that the main storage compartment of gelatinase is within the peroxidase-negative specific granules.