Evidence is presented that at least 17% of microscopically normal bone marrow samples obtained from patients with undifferentiated lymphomas contain occult tumor cells. Of 19 microscopically normal bone marrow samples tested, continuous tumor cell lines were obtained from 4. A tumor cell origin was confirmed by the presence of an 8;14 chromosomal translocation in each case, and HLA typing confirmed the patient origin of the cell line. In two other patients, direct cytogenetic examination of microscopically normal bone marrow samples revealed karyotypes containing 8:14 translocations or a 14q+ chromosome. These findings indicate that undifferentiated lymphomas are often more widespread than is clinically appreciated. The presence of submicroscopic marrow involvement is also of significance to the design and analysis of treatment protocols involving autologous marrow infusion.