Monocyte chemoattractant protein 1 mediates glomerular macrophage infiltration in anti-GBM Ab GN. Monocyte chemoattractant protein 1 (MCP-1) is a C-C chemokine with potent monocyte chemotactic and activating properties that may contribute to glomerular macrophage infiltration in anti-GBM Ab GN. We have previously reported increased glomerular steady state expression of MCP-1 mRNA relative to GAPDH mRNA in the heterologous phase of experimental anti-GBM Ab GN. In this report, we expand upon these data by demonstrating that the increase in MCP-1 mRNA correlated with MCP-1 protein expression at 24 hours that was determined with an ELISA (2069 ± 147 pg/mg glom lysate). This increase in MCP-1 expression was associated with glomerular monocyte/ macrophage infiltration which peaked at 24 hours (8.2 ± 1.0 ED-1 cells/glom). The site of MCP-1 mRNA production was localized by combining immunohistochemistry with in situ hybridization. The majority of cells which expressed MCP-1 mRNA at three hours were intrinsic glomerular cells, while 55% of the cells that expressed MCP-1 mRNA at 15 hours were monocytes/macrophages. To determine if MCP-1 affected glomerular macrophage infiltration, rats with α-GBM Ab GN were administered a polyclonal neutralizing Ab to rat MCP-1. This resulted in a 38% decline in glomerular macrophage infiltration (3.3 ± 0.3 vs. 1.8 ± 0.2 ED-1 cells/glom, P = 0.0001) that was associated with a 45% reduction in urinary protein excretion (260 ± 53 vs. 162 ± 46 mg/d, P = 0.0001). These data demonstrate an important role for MCP-1 in the pathogenesis of glomerular macrophage infiltration in anti-GBM Ab GN.