In order to clarify the function of human S100 beta-positive T-cells, S100 beta-positive T-leukemia cells (S100 beta TLC) were examined in vitro. S100 beta TLC were obtained from the peripheral blood of a patient with S100 beta-positive T-cell leukemia and enriched by an E-rosetting method. Two dimensional flow cytometric analysis indicated that the vast majority of the E-positive fraction were S100 beta TLC expressing CD3 and CD8 antigens. Although S100 beta TLC expressed CD3 antigen, they were negative for the alpha/beta and gamma/delta T-cell antigen receptor (TCR) defined by monoclonal antibodies (mabs) WT-31 and delta TCS-1, respectively. It was speculated that S100 beta TLC initially expressed alpha/beta TCR but lost it during malignant transformation. When S100 beta TLC were cultured for 24 h, they acquired cytotoxic activity towards various NK-sensitive cell lines including K-562, Molt-3 and CEM-CCLF, but did not exhibit lysing activity towards NK-resistant cell lines including Raji, Daudi and MT-1. Despite the NK-activity of cultured S100 beta TLC, they lacked the morphological features of large granular lymphocytes (LGL). S100 beta TLC did not exhibit lymphokine-activated killer (LAK) activity. When S100 beta TLC were cocultivated with NK-sensitive cells or NK-resistant cells, they selectively bound to NK-sensitive cells, indicating that they lysed target cells by cell-to-cell contact. The finding that S100 beta TLC lacked TCR molecules and their NK activity was not inhibited by mabs reactive with the CD3-TCR complex indicated that the CD3-TCR complex was not involved in their target recognition.(ABSTRACT TRUNCATED AT 250 WORDS)