A rapid cellular response occurs when human neutrophils are challenged with the zymosan particle. Monolayers of neutrophils adhering to glass were incubated with opsonized zymosan at 37 ~ for 15 see to 3 min, fixed and prepared for observation by SEM, immunofluorescence or TEM. At 15 sec zymosan particles are seen by SEM in contact with the cell surface and by 30 sec partial engulfment into open cavities was apparent. The cavity, into which a zymosan particle is engulfed, forms by extension of lamellipodia around and over the particle. The zymosan particle is sequestered in an intracellular vacuole or phagosome by 1 min. At 3 min neutrophils often show bulging profiles of the internalized zymosan and a loss of surface projections. During early phagocytosis, simultaneous staining with contrasting fluorescent antisera to granule constituents, myeloperoxidase and lactoferrin, reveals granule antigens within the cavity of the forming phagosome. Sections through the cells examined by TEM after 1-3 min incubation with zymosan show granule contents in the phagosome peripheral space which retains connection to the extracellular environment.