Expression of monocyte chemoattractant protein-1 (MCP-1) in human glioma cell lines and surgical specimens was studied by Northern blot analysis, reverse-transcription polymerase chain reaction, in situ hybridization, and immunohistochemistry. The samples tested consisted of 11 human glioma cell lines and eight specimens of human malignant glioma (seven from glioblastomas and one from a malignant ependymoma). Messenger ribonucleic acid (mRNA) of MCP-1 was detected by either Northern blot or reverse-transcription polymerase chain reaction analysis in all cell lines and tumor specimens examined. In vivo expression of MCP-1 mRNA and protein was found predominantly in glioma cells with large and pleomorphic nuclei rather than in areas of small nucleated glioma cells. Adjacent brain tissue did not produce a significant level of MCP-1 mRNA or protein. Tumor vessels with endothelial proliferation expressed a moderate level of MCP-1 protein. Macrophages were found among the glioma cells, and the degree of macrophage infiltration was grossly correlated with the level of MCP-1 expression. The study results suggest that MCP-1 produced by the glioma cells may mediate macrophage infiltration into the glioma tissue.