The purpose of this study was to explore further an in vitro system to evaluate the interaction between cancer cells and normal cells. In the tumor-induced marrow cyto toxicity assay, 59Fe-and 5tCr-labeled bone marrow cells were cocultured with different typesof cancer cells. Follow ing an 18-hr incubation, the mixed-cell cultures were cen trifuged, and the 59Feand 5tCr in the supernatant solution (releasedby cell death) were compared to the total radio activity and expressed as a release index. In the presence of cocuhtured Novikoff or Yoshida hepatoma cells, the release of isotopes by rat bone marrow cells increased linearly with increasing tumor cell concentrations. With equal numbers (1 x 107/ml)of cancer cells and 59Fe-labeled marrow cells, the marrow releaseindices with Novikoff and Yoshida hepatoma cells were 36.1 Â±0.1% (S.D.) and 38.6 Â± 1 .0%, respectively, as compared to a spontaneous mar row release index of 27.4 Â±0.1%. Similar results were obtained when mouse teratocarcinoma cells were cocul tured with syngeneic mouse marrow cells. Transformed rat fibroblasts were also able to induce the cytolysis of normal rat marrow or fibroblast target cells. Normal rat spleen cells, blood lymphocytes and fibroblasts were inactive as effector cells in the tumor-induced marrow cytotoxicity assay.Activated macrophageswhich havedemonstrated a cytolytic capacity in other assay systems were able to induce marrow cell lysis. However, phytohemagglutinin stimulated lymphocytes were not cytotoxic to normal mar row. These experiments suggest that the capacity to induce the releaseof 59Feand 51Crfrom labeled marrow cells is a common property of transplanted cancer cells. We propose that the tumor-induced marrow cytotoxicity assaywill be a useful experimental technique to enhance our understand ing of the mechanismsof cancer invasion.