I mmunofluorescent techniques were utilized to identify the types of infiltrating lymphocytes adjacent to human malignant tumors arising from a wide range of anatomic sites. 24 of 29 primary tumors and 5 of 8 metastatic lesions showed varying degrees of lymphocytic infiltration. T cells predominated in the infiltrates in primary tumors (mean 80%, range 50-100%) and this pattern was evident regardless of anatomic site or the presence or absence of metastatic spread. By contrast, B cells predominated at the margins of three of five tumor metastases. Mononuclear cells bearing the Fc receptor were not a prominent component of the infiltrates associated with either primary tumors or metastases, but tumor cell binding of fluoresceinated IgG aggregates was observed in 12 of 29 primary tumors. A significant reduction in peripheral blood T cell numbers occurred in a third of the patients studied. This decrease was not clearly related either to the extent of local tumor T cell infiltration or to the presence of disseminated disease. These preliminary findings provide a descriptive analysis of the local and systemic distributions of immunocompetent cells in cancer.