MRP8 and MRP14, two S100-like calcium-binding proteins, are expressed during differentiation of monocytes/macrophages. Both assemble to different noncovalently associated complexes that are supposed to represent the biologically active states. The present study was intended to investigate the molecular basis of macrophage heterogeneity with respect to expression and complex formation of MRP8 and MRP14 during acute and chronic rejection of renal allografts. First, specificity of antisera and mAbs to be directed against MRP8, MRP14, or MRP8/MRP14 heterodimers was determined by immunocytochemical and Western blot analyses of L132 fibroblasts (co-)transfected with MRP8 and/or MRP14 cDNA. Then, immunohistochemical analysis of biopsy specimens obtained from kidney allografts after acute rejection was performed, revealing a parallel expression of MRP8 and MRP14 with coincident MRP8/MRP14 heterodimer formation in infiltrating monocytes. In contrast, chronic allograft rejection was characterized by a subpopulation of monocytes defined by the absence of MRP8/MRP14 complex formation despite expression of MRP8 and MRP14 monomers. Double-labeling experiments showed that this was due in part to differential expression of MRP8 and MRP14 in infiltrate macrophages of chronic rejection. The data presented demonstrate for the first time differences in MRP8/MRP14 complex assembly by infiltrating monocytes in situ. These seem to be of pathophysiological relevance since complex formation defines subpopulations of monocytes associated with distinct pathways of immunological reactions. Differences in the mode of calcium-dependent signaling may, therefore, be of importance for understanding the molecular basis of macrophage heterogeneity during acute and chronic allograft rejection.