Histologic grading of the histiolymphocytic reaction in nodules of T241 or B16 murine tumors in 94 C57BL/6J mice was performed at various time intervals post-inoculation. The infiltrate occurring in the first four days was generally sparse and no significant difference was observed between two different dose control inocula, suggesting a weak primary localization. Comparison between control inocula, and challenge inocula in mice harboring the same tumor for 10 days, showed a somewhat denser infiltrate initially in the challenge inocula. Despite the immunogenicity of these tumors, as shown by concomitant immunity, lymphocytic infiltration was generally sparse, consistent with an hypothesis of deficient localization of immunocompetent cells at tumor sites.