Natural killer (NK) cytotoxic activity during the growth of melanotic (Ma) and amelanotic (Ab) variants of Bomirski hamster melanoma, in the blood and the spleen, was examined. The melanoma variants differed in their growth rate and metastatic pattern. Ability to form conjugates by effectors with targets and cytotoxic effects were compared. As targets K562 cells and tumor cells were used. It was found that during the growth of Ma melanoma activity of NK cells was not changing but the sensitivity of tumor cells was decreasing. However, during growth of Ab melanoma both NK activity and NK sensitivity were reduced.