It is known that the HTLV-I-transformed cell line MT4 releases chemotactic activity for monocytes spontaneously. The MT4 monocyte chemoattractant was purified to homogeneity and sequencing of 25 amino acids revealed identity with the C-C chemokine macrophage inflammatory protein-1α (MIP-1α/LD78). An anti-MIP-1α/LD78 rabbit antiserum substantially inhibited chemotaxis of the MT4 chemoattractant. MT4 cells constitutively expressed MIP-1α/LD78 but not the C-C chemokines MCP-1, RANTES, and MIP-1β/Act2 and the C-X-C chemokines IL-8, groα, and groβ. MT4-derived MIP-1α/LD78 was active on monocytes but was a weak chemoattractant for polymorphonuclear leukocytes. Thus, MIP-1α/LD78 is a major monocyte chemoattractant released by HTLV-I-transformed T cells. Expression of MIP-1α/LD78, a leukocyte chemotactic and myelosuppressive molecule, may play an important role in the manifestations of HTLV-I-related diseases.