SUMMARYThe phenotypic characterization of the non-productive, Marek's disease virus-transformed, lymphoblastoid cell line, JMV-1 and the immunological effects of its cell-free culture supernatant were examined. It was verified that the JMV-1 cell line is an activated T-helper cell line that bears antigens indicative of T-cell activation, and it is not a natural killer or cytotoxic Tcell line. Furthermore, the JMV-1 cell line was shown to be unique among Marek's diseasetransformed cell lines in that JMV-1 cells secrete factors that are able to stimulate in vitro natural killer cell cytotoxicity against the avian tumour cell line, LSCC-RP9, in a 4-h assay. Culturing spleen cells in the presence of JMV-1 supernatant for 4 h stimulates a significant increase in NK cell populations and in la-bearing cell populations, as detected by monoclonal antibody staining. These findings suggest that the protective effects against the parasitic and virally-induced lymphoproliferative poultry diseases, which have been credited to the JMV-1 cell line and to its cell-free culture supernatant, are in part due to lymphokine activation of NK cell cytotoxicity.