Ultrathin cryosections are the most favorable material for localization of intracellular antigens with particulate probes (e.g., colloidal gold) in post-embedding immunocytochemistry. Cryosections are prepared under the most benign conditions as compared to embedding samples in various kinds of plastic media. Typically, higher labeling efficiencies can be achieved with ultrathin frozen sections than with plastic sections.We have utilized human neutrophils, the most abundant type of leukocyte, as a model system to explore labeling procedures for ultrathin cryosections. These labeling procedures can serve as alternative or complementary approaches to the traditional colloidal gold label. Neutrophils are characterized by the presence of numerous granules in their cytoplasm. For many years, the model for neutrophil structure has held that there are two granule types in these cells, the socalled azurophilic and specific granules. We have described an additional cytoplasmic compartment with unusual properties in neutrophils. This compartment is characterized by the presence of the enzyme alkaline phosphatase (Fig. 1).