One of two macrophage inflammatory protein variants (MIP-1a, MIP-1b) with 60% sequence similarity. Each are CC chemokines predominantly secreted by macrophages following stimulation with bacterial endotoxin, though they are also expressed under many other circumstances including viral infections.

CCL3 is chemotactic for granulocytes, including neutrophils, basophils, and eosinophils. MIP-1a will induce the synthesis of IL-1, IL6, TNF. It is usually found elevated along with these cytokines as well as other pro-inflammatory cytokines and chemokines (eg. MCP-1, RANTES, IL-8, etc.)