In an attempt to explore new or alternative therapy for refractory cutaneous T cell lymphoma, a patient with advanced disease refractory to conventional chemotherapy was treated with anti-thymocyte globulin. The therapy was associated with tolerable and manageable toxicities. Phenotypic analysis of neoplastic cells in blood and tissue was used to follow the disease response during the trial. The therapy resulted in resolution of skin lesions and disappearance of circulating cells with neoplastic phenotype. Marked improvement in skin infiltrate and restoration of normal skin architecture seen in skin biopsy specimens after anti-thymocyte globulin were prominent correlative findings. Lymphocytes with normal T cell phenotype appeared after anti-thymocyte globulin therapy, although their mitogenic response was subnormal. The infused anti-thymocyte globulin was found to localize in the skin and bind the neoplastic cells in circulation. The anti-thymocyte globulin-bound cells, with a half-life of 4.9 days, could still be detected up to 14 days after completion of anti-thymocyte globulin therapy. © 1985.