Structure/activity relationships underlying the function of monocyte chemoattractant protein-1 (MCP-1) have been probed by site-directed mutagenesis and indicate that the N-terminus of MCP-1 plays a critical role in activating the MCP-1 receptor, especially aspartate-3. However, a monocyte chemoattractant motif analogous to the ELR sequence of neutrophil-active chemokines has not yet been identified. Amino acids whose side chains project from one face of the first beta-pleated sheet of MCP-1 are also involved in biological activity as is arginine-24. Among C-C chemokines, position 24 is occupied by arginine only in MCP-1, -2, and -3, suggesting that arginine may be a substitution specific for monocyte chemoattractants. Several MCP-1 variants antagonize wild-type MCP-1 activity, the most potent being N-terminal deletion variants. One such variant lacking amino acids 2-8 (called 7ND) inhibits monocyte chemotaxis in response to wild-type MCP-1, but not in response to chemically crosslinked MCP-1 homodimers. This indicates that N-terminal deletion variants exert their effects as dominant negative inhibitors, which implies that MCP-1 activates its receptor as a dimer. This has profound biological implications and also suggests that the dimer interface may be a target for MCP-1 inhibitors. Finally, mutagenesis has demonstrated that murine MCP-1 consists of two domains. An N-terminal domain colinear with human MCP-1 contains all of murine MCP-1's chemoattractant activity. A C-terminal domain of 49 amino acids, which is rich in serine and threonine, contains an extensive amount of O-linked carbohydrate that accounts for 50% of murine MCP-1's apparent molecular size.