We investigated the density distribution of neutrophils in peripheral blood of allergic subjects. We divided neutrophils into four groups: fraction 1 (density greater than 1.085), fraction 2 (1.081 less than density less than or equal to 1.085), fraction 3 (1.077 less than density less than or equal to 1.081) and fraction 4 (density less than or equal to 1.077). The percentage of neutrophils in fraction 2 in allergic rhinitis (AR) subjects or asthmatics was lower than that in normals (p less than 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 less than 0.01; fraction 4, p less than 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(-7) M) changed the density of neutrophils (p less than 0.01), which were inhibited by WEB 2086 (p less than 0.05). Furthermore, granulocyte-macrophage colony-stimulating factor changed the density of neutrophils (p less than 0.01). These findings suggest that biological agents may activate neutrophils and convert their density resulting in neutrophils with lower density in allergic subjects.