Freshly isolated human thymocytes lack the NKH1 antigen and the ability to lyse target cells without major histocompatibility complex restriction. Short-term culture of human thymocytes in interleukin (IL) 2 results in the generation of non-major histocompatibility complex-restricted effector cells, all of which express NKH1. The mechanism by which these cells appear in culture has yet to be elucidated. In the present studies, we developed thymocyte clones and performed a molecular analysis of T cell receptor gene rearrangements to demonstrate that the expression of NKH1 antigen is induced on the surface of NKH1- thymocytes in the presence of IL2. In addition, we were able to show that the NKH1+ fraction consistently displayed an increased proliferative response to similar concentrations of IL2 when compared to NKH1- cells, for both clonal and polyclonal populations of thymocytes. Taken together, these studies demonstrate that the initial appearance of the NKH1 antigen following thymocyte culture in the presence of IL2 results from the induction of NKH1 expression on NKH1- thymocytes, while the subsequent predominance of this cell type also results from an enhanced proliferative response to IL2 which coincides with NKH1 expression.