Immunogold electron microscopy of rabbit PMNs stained with antibodies against p15 or BPI and visualized with protein A–coated gold particles. Rabbit PMNs were incubated with an MoAb against BPI (A) or guinea pig antiserum raised against rabbit p15s (B) as described in Materials and Methods. Single arrows point to primary granules and double arrows point to secondary granules; nuc, nucleus.