Previously, we reported that high concentrations of eosinophils in human colonic carcinomas are associated with better prognoses, that sections taken 1 cm remote from (deep to) the margin of tumor (SRM) and sections contiguous to the margin (SCM) of tumor and adjacent uninvolved colon contain significantly different concentrations of eosinophils, and that concentrations of eosinophils in SCM and SRM are both useful and complementary for the prediction of prognosis. As a first step towards studying the ecology of the eosinophil in colonic carcinoma and with the goal of identifying other kinds of cells that might be useful for the prediction of prognosis, we counted cells in SCM and SRM that expressed histochemically demonstrable acid phosphatase, a-naphthyl butyrate esterase, and peroxidase. The tumors of patients with and without metastases at the time of resection of the primary tumor contained different (P = 0.0314) concentrations of cells with histochemically demonstrable a-naphthyl butyrate esterase in SCM but not in SRM. In contiguous 1- to 2-/*m sections, morphologically macrophage-like cells with histochemically demonstrable acid phosphatase and cells with histochemically demonstrable a-naphthyl butyrate esterase were found to be present in different concentrations both in SCM (P < 0.01) and in SRM (P < 0.01); i.e., these phenotypic markers appear to identify different subpopulations of macrophages in tumors. In contrast to our previous study of human pulmonary alveolar macrophages, examination of sections stained sequentially for these phenotypic markers that are commonly used for the identification of macrophages in tumors revealed numerous cells in the same sections that expressed histochemically demonstrable acid phosphatase (red) but not a-naphthyl butyrate esterase (brown) and vice versa. Several of these markers promise to be useful and complementary for the prediction of prognosis. © 1985, American Association for Cancer Research. All rights reserved.