Mononuclear cells from peripheral blood leucocytes of foot-and-mouth disease (FMD) vaccinated cattle underwent blast transformation after in vitro culture with purified, inactivated, 146 S FMD virus antigen. From the prolonged culture of blast cells in medium with Interleukin-2 (5-10 U/ml), CD 45-positive effector cells were derived, which showed a potent, non MHC-restricted activity against virus-infected cells, the extent of which was inversely correlated with multiplicity of infection (MOI). Extensive characterization of effector cells by means of monoclonal antibodies (MAbs) in flow cytometry, lysis of various cell populations with MAbs and complement, Fc receptor analysis and 51Cr release assays indicated that the isolated cells share some phenotypic and functional features of the human and murine Large Granular Lymphocyte/Natural Killer (N.K.) lineage.