Studies are reviewed which, in part, identify T lymphocytes and macrophages as the principal inflammatory cell types found in regressing and progressing Moloney sarcomas. Isolated in vitro from regressing neoplasms, each of these cell types was cytologic. The T cells killed in an antigenically specific manner, while the macrophages lysed target tumor cells without regard to their antigenic composition. The capacity of each of these effector cell types to mediate cytolysis was impaired if they came from large, progressing tumors. In addition to functional differences, mononuclear inflammatory cells were distributed differently in the 2 types of tumors, regressing sarcomas being infiltrated throughout in contrast to progressing tumors, which somehow relegated inflammatory elements to the margins of the neoplasms. The studies which are reviewed suggest that functional and distributional alterations in cytolytic inflammatory cells may predispose mice to Moloney sarcoma progression, rather than regression, and underscore the importance of comprehensively evaluating inflammatory cell responses within tumors as part of the analysis of host-tumor interrelationships.