In a young girl with a severe combined immunodeficiency, the presence of circulating maternal T lymphocytes was proven by HLA typing. Manifestations of skin graft vs host disease were associated with the persistence of maternal cells. The patient received an HLA identical bone marrow transplantation from her brother without any conditioning. The bone marrow transplantation was quickly followed by a transient and dramatic increase in skin lesions associated with fever and the finding of a high number of circulating lymphocytes and eosinophils. Lymphocytes were shown to be of donor origin and exerted a spontaneous cytotoxic activity toward maternal cells. This activity progressively disappeared within 90 days, whereas maternal cells were no longer detected in patient's blood, and skin graft vs host disease was resolved within 8 wk. Cytotoxic activity was proven to be mediated by donor T lymphocytes specific for the mother's HLA antigens. The cytotoxic activity was demonstrated to be specific for the HLA class I molecules of the mother not shared with her daughter (HLA A1, B17) as shown by the use of a series of HLA typed cells as targets. In addition, cold K562 target cells did not block the cytotoxic activity, and the kinetics of the cytotoxic activity did not correlate with that of natural killer activity emergence after the bone marrow transplantation. Patient's serum did not contain antibodies toward maternal specific HLA class I antigen. Cytotoxic activity was totally blocked by anti-T3 monoclonal antibodies and partially by anti-T8 and anti-T4. It is thus likely that donor origin cytotoxic T lymphocytes were promptly activated after bone marrow transplantation and provoked the elimination of the maternal graft after a transient exacerbation of graft vs host disease manifestations. This observation represents one of the first examples of the possible role in vivo of allogeneic cytotoxic lymphocytes in humans.