Neutrophil (PMNL) function is influenced by factors released by other immune cells during the course of the immune response. We investigated the effect of neutrophil cell density and the effect of supernatant of the phagocytosis assay on the phagocytosis activity of PMNLs. The measurements were carried out with naive (control) PMNLs of healthy donors and with PMNLs obtained from patients with severe tissue injury. Phagocytosis index (FI) of PMNLs was determined at cell densities of 7.5x10 5 /ml and 15x10 5 /ml. E. coli phagocytosis of heparinized whole blood from healthy donors and patients with severe tissue injury was measured and evaluated at three different cell densities (normal, half, and double densities) by flow cytometry. Supernatants of phagocytosis assays of either control or trauma (ISS >18) patient PMNLs were added to the assay suspensions of control and trauma PMNLs. An increase in cell density of healthy donor PMNLs increased yeast phagocytic activity. In cases of tissue injury, PMNLs showed increased phagocytic activity at lower cell densities. E. coli phagocytosis was increased with the increase of cell density, and tissue injury PMNLs were more active at each cell concentration compared to naive cells. Polytrauma supernatants in most cases inhibited, while healthy supernatants mostly increased the yeast phagocytosis of healthy and trauma PMNLs. These results reinforce the idea that primed PMNLs in the presence of microbial agents produce factor(s) which inhibit some of the cell's antimicrobial functions contributing to immunedysfunction, while unprimed PMNLs produce factor(s) which facilitate antimicrobial countermeasure. These results also demonstrate that reduced phagocytosis of tissue injury primed PMNLs is not due to cytoskeletal changes but to the humoral environment.