Elicited murine neutrophilic polymorphonuclear leukocytes (PMN) were fractionated by Percoll density gradient centrifugation into high density (HD) and intermediate density (ID) populations. As described in the accompanying article HD- and ID-PMN appear to represent "resting" and "activated" cell populations, respectively. Consistent with this possibility, histochemical and biochemical evidence suggested that ID-PMN were degranulated compared to HD-PMN. Myeloperoxidase (MPO) in the ID-PMN population showed increased sensitivity to inhibition by 3-amino-1,2,4-triazole, and HD-PMN exhibited a 2-3-fold increase in chloride and iodide oxidation per unit of MPO activity compared to ID-PMN. When HD-PMN were induced to degranulate in vitro, the remaining cell-associated MPO displayed enzymatic properties characteristic of the activity associated with ID-PMN. The mechanism of this phenomenon was also investigated in vitro using purified human peripheral blood PMN and the synthetic chemotactic peptide N-formyl-methionyl-leucyl-phenylalanine. Differences in cell-associated MPO activity were shown to be related to selective exocytosis of enzymatically and chromatographically distinct forms of the enzyme. These data indicate that, in addition to the well known selective exocytosis of specific and azurophilic granules induced by various agents, selectivity may also occur at the level of enzymatically distinct forms of a particular granule enzyme. Moreover, our observations provide further evidence that density differences may be utilized to fractionate and study the generation of functionally distinct subpopulations of PMN that arise in vivo as well as in vitro following exposure to various stimuli.