Moloney murine leukemia virus (Mo-MuLV) is a thymotropic and leukemogenic retrovirus which causes T lymphomas. Recently, Mo-MuLV has been shown to trans-activate cellular genes. Monocyte chemoattractant protein-1 (MCP-1) is a chemokine which can promote the migration and diapedesis of monocytes and lymphocytes, as well as inducing metastasis of lymphomas. Here we demonstrate that introduction of Mo-MuLV or the MuLV LTR alone, transiently or stably, into Balb/c-3T3 cells or HeLa cells resulted in 9-11 fold increases in MCP-1 transcripts. This trans-activation of the MCP-1 gene by the Mo-MuLV LTR is independent of the physical location of the MCP-1 gene or of the LTR, occurring whether the LTR or the MCP-1 gene is integrated in the genome or transiently expressed. Immunoblot analysis using an anti-MCP-1 polyclonal antibody showed that the expression of the MuLV LTR in HeLa cells also induced the appearance of the MCP-1 protein. Boyden Chamber analysis demonstrated that the MCP-1 chemotactic activity produced by HeLa cells with an integrated MuLV LTR was elevated by 11 fold and that neutralizing antibody to human MCP-1 abrogated monocyte migration in response to MuLV LTR expression. Promoter deletional analysis showed the LTR responsive cis-acting element in the MCP-1 promoter is located between -141 and -88. Deletion of this region abolished the trans-activation of MCP-1 by the LTR. These LTR-mediated activations of a chemotactic and inflammatory cytokine may be relevant as mechanisms whereby retroviruses which do not contain oncogenes can induce neoplasia.