Ectromelia virus-specific cytotoxic T (Tc) cell precursors were present in the draining popliteal lymph node of all strains of mice tested at 2 to 3 days after footpad inoculation of a high dose (10(5) p.f.u.) of the virulent Moscow strain of ectromelia virus. To detect this response it was necessary to culture lymph node cells from infected mice in the presence of T cell growth factors and to use the more sensitive neutral red assay for measuring cytotoxicity. Cells with lytic activity were virus-specific, major histocompatibility complex-restricted TC cells. C57BL/6J resistant mice, which express a single dominant gene conferring innate resistance had virus-specific TC cell precursors 1 to 2 days sooner than did susceptible BALB/b mice. This TC cell-mediated immune response early after infection could account for the barrier to virus dissemination known to operate 1 to 2 days after infection to slow virus passage into the lymphoreticular system.