AbstractOmission of L‐glutamine (Gln) from the culture medium decreases tyrosinase activity and increasesγ‐GTP activity in cultured melanotic B‐16 melanoma cells; inclusion of this amino acid in the medium shows the reverse effects. Its absence, however, decreases theγ‐GTP activity in non‐pigmented cells like HeLa cells, fibroblasts and amelanotic melanoma cells and its presence induces a dose dependent increase in this enzyme activity in these cells. The decrease ofγ‐GTP activity in the melanotic melanoma cells by Gln is attributed to increased production of dopa and its derivatives by these cells, as evidenced by our results. Our data also indicate the inhibitory effects of dopa and several other dopa derivatives known to be produced by the melanoma cells on isolated bovi