A quantitative investigation into the mechanism of neutrophil phagocytosis of opsonized microspheres possessing well defined dimensions was undertaken. Three aspects were documented: membrane conservation, cell adhesion to the spheres, and active cell cytoplasmic projection around the microspheres. The physical act of internalizing a particle by a cell involves a reduction in its plasma membrane area and an increase in its volume. As a consequence, a cell can internalize only a finite number of particles. A store of membrane area exists on cytoplasmic granules and may be recruited during phagocytosis. Previous measurements of neutrophil membrane area and volume served as a basis for estimates of the maximum number of internalized microspheres. A comparison with experimental prediction based on membrane conservation and degranulation agrees within 10% for a range of microsphere diameters, from 0.5 to 8 microns. This suggests that the limitation for additional particle uptake in the population of engorged neutrophils is the lack of excess plasma membrane area. In a random population of neutrophils, there was a sub-group, approximately 40%, which could no longer phagocytose before depleting their membrane stores. Several aspects of the engulfment process were investigated to elucidate the cause of this phagocytosis deficiency. It could be shown by single cell observation that these cases were associated with a lack of pseudopod projection, although adhesion was still evident between the cell membrane and the microspheres.