Natural killer-cell activity for K562 target cells was measured in 13 patients with severe combined immunodeficiency before bone marrow transplantation. Both unseparated peripheral blood mononuclear cells and sorted cell subsets (B73.1 positive, B73.1 negative, OKT3 positive, OKT3 negative) were tested. Heterogeneity of natural killer-cell activity was observed, the level of which did not correlate with the usual subdivision of severe combined immunodeficiency, as defined by a scientific group on immunodeficiency for the WHO. In those patients in whom natural killer-cell activity was observed in unseparated cells, testing of this function in sorted cell subsets revealed some unexpected findings. In three of four patients, B73.1-negative cells showed remarkable natural killer-cell activity. In addition, in one of these patients, no activity was observed in the B73.1-positive cells.