Transplantable murine tumors, such as the squamous cell carcinoma growing in C3H mice, include a significant proportion of normal cells. The nature of these cells, their locations relative to the blood supply, their oxygenation status, and ability to incorporate 3H-thymidine were examined by sorting cells staining positive or negative with fluorescein isothiocyanate-conjugated goat antimouse IgG. Of the cells that were recovered from this tumor, 39% ± 19 (n = 25) were IgG + cells, and this percentage was independent of tumor sizes greater than 0.2 g and less than 1 g. Cells staining positive (i.e., containing the Fc receptor for the IgG molecule) were diploid, non-clonogenic cells. More than 95% of the cells that bound the antibody rapidly phagocytosed latex microspheres, indicating that the host cells in the tumor were primarily macrophages. The negative-staining cells were more than 90% near-tetraploid. Macrophages were distributed randomly through the tumor cord. Both tumor cells and macrophages incorporated 3H-thymidine, with greater incorporation by larger cells close to the functional tumor blood vessels. Conversely, in cells distant from the blood supply, binding of the hypoxia probe misonidazole was enhanced in both macrophages and tumor cells, and the rates of metabolism of misonidazole were similar for both. Removing macrophages prior to plating tumor cells in vitro had no obvious effect on tumor cell viability after treatment of mice with x-rays or Adriamycin®.