Previously, a subset of T cells co-expressing the NK cell antigen CD56 has been described. These CD3+CD56+ cells are rare in peripheral blood collections and have been poorly characterized. We have developed culture conditions which allow for the rapid expansion of CD3+CD56+ cells. The protocol for cellular expansion includes the addition of interferon-gamma on day 0, interleukin-1, interleukin-2 and a monoclonal antibody against CD3 on day 1 to peripheral blood lymphocytes. Cells of the CD3+CD56+ phenotype increased up to 6000-fold using this protocol after 16 d in culture. These cells have been characterized by flow cytometry and have been found to express the alpha, beta T cell receptor, co-express the CD5 and CD8 antigens and do not express the CD16 antigen. Morphologically, these cells cannot be distinguished from NK cells. CD3+CD56+ killer cells lyse a variety of tumour cells with intermediate activity between CD3-CD56+ NK cells and CD3+CD56- T cells.