A synthetic peptide, AVLPRSAKEL (LU10), the N-terminal amino acid sequence of chemotactic protein (LUCT/IL-8), showed chemotactic activity to polymorphonuclear leukocytes (PMN) with an ED50 of 5 nM for comparable to that of LUCT. Native LUCT and LU10 specifically induced the phosphorylation of 64 kD protein of PMN, and serine residue in the 64 kD protein was major phosphorylated amino acid. Furthermore, native LUCT enhanced the release of myeloperoxidase and beta-glucuronidase from PMN in the presence of cytochalasin B and FMLP, but LU10 did not. These results strongly suggest that the active site for both chemotactic stimulation and 64 kD protein phosphorylation is localized on the sequence of N-terminal 10 amino acids of LUCT.