Leukotactin-1 (Lkn-1), a human CC chemokine that binds to both CC chemokine receptor (CCR)1 and CCR3, is distinct from other human CC chemokines in that it has long amino acid residues preceding the first cysteine at the NH(2)-terminus. Serial deletion studies showed that at least three amino acid residues, alanine-alanine-aspartic acid (A-A-D), preceding the first cysteine at the NH(2)-terminus are essential for the biological activity of Lkn-1. Point mutation and deletion studies for the three amino acids were performed in the present study. Substitutions of the first alanine residue with other amino acids did not cause significant loss of biological activities. Deletion of the third amino acid, aspartic acid, resulted in more than 100-fold loss of the activity. Deletion of two amino acids, alanine-alanine (A-A) or alanine-aspartic acid (A-D), resulted in almost complete loss of the activity. Loss of agonistic activity by deletion of two amino acids was due to impaired binding to CCR1. These results identify that alanine-aspartic acid residues preceding the first cysteine at the NH(2)-terminus are essential for the binding and biological activity of Lkn-1.