We examined the expression of the MCP-1/JE gene in freshly prepared human monocytes and again after either in vitro or in vivo maturation of these cells. We show that previously unstimulated blood monocytes of healthy individuals prepared by adherence procedures display high levels of MCP-1/JE mRNA and protein. Monocytes that were not previously exposed to activational plastic surfaces but were separated from other blood cells by counter-flow centrifugal elutriation expressed severalfold lower MCP-1/JE transcript and protein levels. Treatment of these cells with endotoxin was associated with the down-regulation of MCP-1/JE mRNA and protein levels. In contrast, exposure to IFN-gamma resulted in increased MCP-1/JE gene expression. During the process of in vitro maturation of monocytes into macrophages, a gradual decrease of MCP-1/JE mRNA and protein expression was noted. Both mature (day 8) blood monocyte-derived culture macrophages and peritoneal macrophages completely failed to express the MCP-1/JE gene, which could be restored after exposure to IFN-gamma and cross-linking of macrophage Fc gamma R with murine solid phase IgG2a mAbs; however, endotoxin failed to induce MCP-1/JE expression in these cells.