The early response gene for monocyte chemoattractant protein 1 (MCP-1) encodes a potent chemotactic factor that is specific for monocytes. To determine whether MCP-1 is involved in macrophage recruitment in cardiac allografts, we studied time-dependent MCP-1 gene and protein expression patterns in the heterotopic, Lewis to F-344 rat transplantation model (by reverse transcription-PCR and immunohistochemistry). There was a significant increase (8- to 12-fold) in MCP-1 gene transcripts in cardiac allografts compared with host hearts at 7, 14, and 28 days after transplantation. This induction was not observed with syngeneic transplants or hosts exposed to the same circulating cells and blood products. The MCP-1 gene product was expressed predominantly by mononuclear cells that double-stained with antimacrophage antibody (ED1) and localized to the interstitial and vascular spaces of the allografts. Immunocytochemical cell counting revealed significant increases in both MCP-1- and ED1-immunopositive cells in 7-, 14-, and 28-day allografts (in comparison with day 0 hearts). The absolute number of MCP-1-positive cells (5-7%) was lower than that of ED1-positive cells (25-34%) at all time points, suggesting that MCP-1-positive cells represent a subpopulation of activated macrophages. The persistent expression of MCP-1 in association with increased macrophage localization suggests that this inducible mediator contributes to the chronic inflammatory response following cardiac transplantation and that it may play a role in the pathogenesis of transplant arteriosclerosis.