Culture supernatants of phytohemagglutinin (PHA)-stimulated human peripheral blood mononuclear cells (PHA-sup) induced monocyte chemoattractant protein-1 (MCP-1) mRNA expression in human neutrophils. MCP-1 mRNA was first detected by Northern analysis at 8 h, and the peak level was detected at 16 h and sustained until 72 h. Cycloheximide and genistein, but not pertussis toxin, inhibited the expression of MCP-1 mRNA. Recombinant tumor necrosis factor α (TNF-α) induced a low level MCP-1 mRNA accumulation in neutrophils, and addition of anti-TNF-α IgG blocked 30–70% of MCP-1 mRNA expression induced with PHA-sup. PHA-sup-stimulated PMN synthesized and secreted 3.1 ± 1.3 ng/5 × 106 PMN MCP-1 within the first 24 h. Hybridization of ³²P-labeled cDNA preparations to an array of human cytokine cDNAs further indicated that MCP-1 mRNA was selectively up-regulated in the late phase after stimulation with the PHA-sup. J. Leukoc. Biol. 65: 671–679; 1999.