M elanoma growth stimulatory activity (MGSA/GROalpha) is a 73 amino acid peptide sharing sequence characteristics with the alpha-chemokine superfamily. MGSA/GROalpha is produced by diverse melanoma cell lines and reported to act as an autocrine growth factor for the cells. We tested the binding of MGSA/GROalpha to melanoma cell lines, Hs 294T and RPMI7951, and found that these cells could bind to MGSA/GROalpha but not to interleukin-8. Recently, we defined a novel hexapeptide, antileukinate, which is a potent inhibitor of binding of alpha-chemokines to their receptors on neutrophils. When antileukinate was added to melanoma cells, it inhibited the binding of MGSA/ GROalpha. The growth of cells from both melanoma cell lines was suppressed completely in the presence of 100 microM peptide. The cell growth inhibition was reversed by the removal of the peptide from the culture media or by the addition of the excess amount of MGSA/GROalpha. The viability of Hs 294T cells in the presence of 100 microM peptide was > 92%. These findings suggest that MGSA/GROalpha is an essential autostimulatory growth factor for melanoma cells and antileukinate inhibits their growth by preventing MGSA/GROalpha from binding to its receptors.