We investigated the density distribution of neutrophils in peripheral blood of allergic subjects. We divided neutrophils into four groups: fraction 1 (density > 1.085), fraction 2 (1.081 < density ≤ 1.085), fraction 3 (1.077 < density ≤ 1.081) and fraction 4 (density ≤ 1.077). The percentage of neutrophils in fraction 2 in allergic rhinitis (AR) subjects or asthmatics was lower than that in normals (p < 0.01). The percentage of neutrophils in fraction 3 and fraction 4 from AR or asthmatics was greater than that in normals (fraction 3, p < 0.01; fraction 4, p < 0.05). In neutrophils from AR subjects (fraction 3), chemotaxis to N-formyl-methionyl-leucyl-phenylalanine or platelet-activating factor (PAF) was enhanced compared to fraction 2. PAF (10––7M) changed the density of neutrophils (p < 0.01), which were inhibited by WEB 2086 (p < 0.05). Furthermore, granulocyte-macrophage colony-stimulating factor changed the density of neutrophils (p < 0.01). These findings suggest that biological agents may activate neutrophils and convert their density resulting in neutrophils with lower density in allergic subjects.