In patients with severe combined immunodeficiency, who have been successfully treated by bone-marrow transplantation, the occurrence of split take has been well documented: whereas myelomonocytic hemopoietic cell lines remain of host origin, the T-lymphoid compartment is of donor origin, while the B-lymphoid compartment may be either of host or of donor origin. We have studied the T gamma cells of two patients, successfully treated by bone-marrow transplantation from donors of the opposite sex, with respect to the sex-chromosome pattern, the binding of OKM1 and OKT3 monoclonal antibody and their K and NK activity. All T gamma cells of both patients were found to be of donor origin. These T gamma cells contained two serologically distinct subpopulations, one OKM1+ OKT3+, the other OKM1+ OKT3-, as was also found in normals. However, the ratio between these two subpopulations is 0.6 in normals, whereas these patients revealed a ratio of 3.0. Furthermore, the patients' T gamma cells displayed a strongly reduced K and NK activity (+/- 30% of normal). We concluded that at least part of the OKM1+ OKT3+ and of the OKM1+ OKT3- T gamma cells are derived from other than the myelomonocytic lineage, presumably from the lymphocytic lineage. The origin of the K and NK active T gamma cells, however, cannot be conclusively determined from these experiments. These findings also imply that the antigen detected by OKM1 should obviously no longer be regarded as exclusively present on myelomonocytic cells.