Based on theoretical considerations and several types of experimental evidence with heterogeneous cell populations it has been proposed that alloreactive T cells and major histocompatibility complex (MHC)-restricted T cells directed to foreign non-NHC antigens represent overlapping subsets. In this report we provide direct evidence for this hypothesis at the clonal level. We have isolated a cloned continuous influenza virus-specific cytotoxic T cell (CTL) line derived from a single (H-2b X H-2d)F1 CTL precursor which simultaneously exhibits H-2-restricted cytotoxicity of influenza A/Japan/305/57 virus in association with H-2Kd and alloreactive cytotoxicity for H-2Kk alloantigen. Cold target inhibition data demonstrate that both MHC-restricted and alloreactive recognition is mediated by the same cell population. In addition to cross-reactivity at the target cell level, we shown that this cloned CTL line can be specifically stimulated to proliferate either by A/Japan/305/57 virus-infected cells expressing H-2Kd or by uninfected cells of the H-2Kk haplotype.