Autologous bone marrow transplantation is used in small cell lung cancer (SCLC) to reverse the hematological toxicity induced by high dose therapy even though the presence of cancerous cells in the graft is potentially dangerous by reinfusion of the disease along with the hematopoietic stem cells. The present studies were undertaken to examine the effectiveness of anti-SCLC rat monoclonal antibodies LCA1 and LC66 plus human complement combined with a derivative of cyclophosphamide (Asta-Z 7557) for the elimination of cancerous clonogenic cells from the graft. In a series of assays conducted with three SCLC cell lines, used alone or mixed with normal bone marrow cells, the addition of Asta-Z 7557 to two cycles of treatment with monoclonal antibodies plus complement results in a 4- to 5-logarithmic reduction of the clonogenic SCLC cells detectable by limiting dilution analysis. This was superior to either treatment used alone. When normal bone marrow was submitted to the same treatment, a median (range) of 44% (15-77%) of the colony-forming unit, granulocyte-macrophage was recovered. These results suggest that the association of immunological (LCA1 and LC66 plus human complement) and pharmacological (Asta-Z 7557) removal methods is effective for purging metastatic clonogenic cells from bone marrow of SCLC patients and could be considered before autologous bone marrow transplantation.