Natural killer (NK) cells are phenotypically defined as lymphocytes expressing the antigens CD56 and mostly CD16 (Fc gamma RIII), but lacking CD3. A small CD3- CD16- CD56+ NK cell subset has been described in normal individuals representing less than 2% of peripheral blood lymphocytes. We analyzed here 70 patients for their reconstitution of the immune system during follow-up after autologous or allogeneic bone marrow transplantation. In 35% of these patients, two different NK cell subsets, namely CD56+dim and CD56+bright cells, were observed. The mean duration of these two subsets after transplant was 4 months. Sixty-five percent of the patients exhibited an increased number of NK cells, but only the typical CD16+ CD56+dim population. The CD56+bright subpopulation represented a particular CD3- CD16- NK subset, with posttransplant frequencies up to 70% of all NK cells and 40% of peripheral blood lymphocytes, respectively. In contrast to normal CD56+dim NK cells, CD56+bright cells coexpressed the activation antigens p75 beta-chain of interleukin-2 receptor (IL-2R), CD2R, and CD26, but were negative for CD16. NK and antibody-dependent cellular cytotoxicity activity of CD56+bright cells was low compared with CD56+dim NK cells. But using IL-2 and interferon gamma, their cytotoxicity could be enhanced even more than in CD56+dim lymphocytes. These different subsets may reflect distinct activation or differentiation steps of NK cells during reconstitution of the immune system. Their differential response to IL-2 may be of functional importance for posttransplant cytokine therapy.