Tissue components of breast cancers grown in female Lewis rats (n = 6) were identified histologically and immunohistochemically. Tracer uptake was studied by quantitative autoradiography 2 hr after an intravenous injection of 100 muCi [3H]FDG. RESULTS;: RMC tumors were glandular, with small foci of necrosis and were surrounded by a thin layer of granulation tissue. Tumors retained approximately 4% of the injected FDG dose (1.9 +/- 0.27 muCi/g). Macrophages numbered 0.5% of total cancer cells (1.2 +/- 1