Interactions of human and rat neutrophils with allogenic and xenogeneic ligands were investigated. Human blood neutrophils reacted equally with human and rat IgG; IgG-dependent adhesion of rat neutrophils (from aseptic peritoneal exudate or blood) was observed only in allogenic system. C3-dependent reactions of neutrophils were observed both in allogenic and xenogeneic systems. The advantage of allogenic combinations in this case was reliable only for rat neutrophils. The relation of obtained results to structural and functional heterogeneity of receptors and ligands is discussed.