The application of hyperthermia to the treatment of neoplastic disease has focused on solid tumors. Since the hyperthermic sensitivity of human B-cell lymphoma cells is not known, we have examined the effect of hyperthermia on the growth of B-cell lymphoma cell lines (Raji and Daudi) in vitro to evaluate the ability to purge tumor cells from normal bone marrow by heat, utilizing a limiting-dilution assay to measure log depletion of tumor cells in a 20-fold excess of normal bone marrow. When exposed at 42 degrees C and 43 degrees C for 120 min, both clonogenic Raji and Daudi cells were dramatically decreased (a 4- to 6-log reduction) with exposure time, while leaving over half of the normal granulocyte-macrophage progenitor cells surviving at 42 degrees C and 10% at 43 degrees C. This high level of lymphoma-cell depletion by heat correlated with that obtained in immunologic and pharmacologic studies. These results suggest that in vitro hyperthermia might be applied effectively for the elimination of residual lymphoma cells in autologous marrow grafts before autologous bone marrow transplantation in B-cell non-Hodgkin's lymphoma.