Group IIA phospholipase A2 (PLA2) is a newly recognized acute phase protein with marked antibacterial properties. We have shown previously that transgenic C57BL/6 J mice expressing human group IIA PLA2 (PLA2+ mice) are more resistant to bacterial infections than nontransgenic C57BL/6 J mice that, among mice, are unusual in that they lack the mouse analogue of group IIA PLA2 (PLA2- mice). To elucidate the possible mechanisms involved in the host response of these mice in bacterial infection, peripheral inflammatory cell responses of PLA2+ and PLA2- mice were studied after i.p. administration of Escherichia coli, E. coli lipopolysaccharide or Staphylococcus aureus. Uninfected PLA2+ mice had higher numbers of lymphocytes and polymorphonuclear neutrophil leukocytes (PMNs) in their blood than PLA2- mice. In PLA2+ mice, the number of PMNs increased in peripheral blood in parallel with the concentration of group IIA PLA2 after the administration of bacteria, whereas these responses were not seen in PLA2- mice. High concentrations of group IIA PLA2 in PLA2+ mice may increase the synthesis of bioactive molecules, such as prostaglandins, which in turn may mobilize PMNs into circulation. Our results support the hypothesis that group IIA PLA2 is an important inflammatory mediator in bacterial infections.