We describe a human lymphoblastoid cell line (LCL), called ZS, that originated spontaneously from the cultures of gamma-irradiated (50 Gy) peripheral-blood mononuclear cells of a normal donor. When injected subcutaneously in sublethally irradiated, splenectomized and anti-asialo-GM1-treated nude mice, ZS cells invaded the lymph nodes, that appeared 10 to 50-fold enlarged in all of the mice tested. Furthermore, ZS cells expressed a typical T-cell surface structure, the CD2 molecule, detectable by a variety of different anti-CD2 monoclonal antibodies (MAbs). However, other T-cell markers were not found, with the possible exception of a truncated messenger of the beta chain of the T-cell receptor and ZS cells could be identified as B cells since they (i) expressed a battery of markers of the resting and activated B cells, (ii) displayed a monoclonal rearrangement of the IgH chain locus and (iii) synthesized IgM K molecules. The Epstein-Barr virus (EBV) genome was detected in ZS cells in approximately ten copies per cell by DNA hybridization techniques. Furthermore, the cells were positive for EBV nuclear antigens (EBNA). Western blotting analysis of EBV encoded antigens demonstrated clear differences with those present in the B 95.8 virus-producer cell line, indicating that ZS cells were not infected by EBV in vitro and that they already harbored the virus in vivo. ZS cells formed colonies in vitro with a high cloning efficiency and displayed chromosomal abnormalities in all of the mitoses (karyotype 47, xy, +13, -14, 8p+, 21p+, +m). In spite of these malignant features, ZS cells expressed the full range of EBV latent proteins as usually do "normal" LCSs and did not have any of the chromosomal abnormalities that juxtapose the c-myc oncogene to one of the genes coding for immunoglobulin molecules.