From a mixed cell suspension of five primary melanomas an aqueous sterile extract was made and added to homologue leucocyte cultures of eight melanoma patients and seven controls. The extract had a blastogenic and mitogenic effect in all cultures. There was no significant difference between the responses of patients and controls. In experiments with an equally prepared extract of normal skin, added to homologue leucocyte cultures of nine persons, none but one had a positive reaction. This indicates, that the motigenic principle of the melanoma extract is not related to the histocompatibility antigen nor caused by chemical or bacterial contamination of the primary cell suspensions. It seems to be derived from the cytoplasmic fraction of melanoma cells. Other experiments to solve the problem were impossible because of lack of material. There are several reports in the literature which by using different methods indicate, that there might be an immunological reaction to melanoma associated principles in healthy persons, too.