When synthetic monocyte chemotactic proteins (MCPs) were tested in a Boyden chamber assay system for eosinophil-chemotactic properties using human eosinophils, MCP-3 was found to be a potent and efficient (percentage input migrating cells) attractant with an ED50 near 2-3 nM. MCP-2 was less potent, exhibiting half maximal chemotaxis at 40 nM. Cross desensitization experiments of eosinophil-chemotaxis revealed that both MCP-3 and MCP-2 desensitized autologous responses. In addition, pretreatment of human eosinophils with natural RANTES desensitized chemotaxis towards MCP-3 as well as MCP-2, whereas pretreatment of eosinophils with MCP-3 desensitized, apart from responses to MCP-3, also responses against RANTES and MCP-2. These findings indicate that possibly both MCP-3 and MCP-2 elicit chemotactic responses via the RANTES-receptor on eosinophils.