By Northern analysis, freshly isolated monocytes contained no detectable mRNA for monocyte chemotactic protein-1 (MCP-1). However, after 4 hours of incubation at 37°C, MCP-1 mRNA was clearly induced in the monocytes and was found to be highly dependent and directly proportional to the monocyte density. The level of MCP-1 mRNA continued to increase, reaching a peak after 22 hours of incubation. After 3 days in culture, MCP-1 mRNA levels had declined substantially and after 8 days were undetectable in the monocytes/macrophages. The amount of MCP-1 protein secreted correlated with the density-dependent increase in MCP-1 message. We hypothesize that the migration of monocytes into inflammatory lesions may be amplified by the density and time-dependent induction of MCP-1.