Résumé : Six patients with cutaneous metastases of malignant melanoma were treated with bacillus Calmette-Guérin (BCG). Rosette formation by sheep erythrocytes surrounding thymus-derived lymphocytes was studied by two methods ("active" and total rosette-forming cells) before and after therapy. Four patients showed good clinical response with regression of some or all skin nodules, and two did not respond. All four responders showed an increase in "active" rosette-forming cells; two also had increases in total rosette-forming cells. One nonresponder had an increase in "active" rosette-forming cells. It is proposed that BCG activates both nonspecific and specific thymus-derived cell populations. Traffic of thymus-derived cells may also occur after administration of BCG. © 1976 Springer-Verlag.