Marrow involvement by small cell lung cancer (SCLC) is detected in 10-23% of patients at initial diagnosis by marrow aspirate and biopsy techniques. To improve the detection and potentially monitor marrow involvement by SCLC we have attempted to concentrate malignant cells with clonogenic potential on a discontinuous density gradient (DDG). The bone marrow from 43 patients with SCLC (36 with histologically negative marrow aspirates and biopsies) were separated on a Ficoll-based DDG. Samples were also separated by conventional Ficoll-diatrizoate (FD) (density, 1.077) gradient sedimentation. The cellular interphase from three fractions (F X) corresponding to specific densities 1.050 (F X 1), 1.055 (F X 2), and 1.060 (F X 3) as well as cells separated by Ficoll-diatrizoate (F X FD) centrifugation were isolated and 2.5 X 10(5) cells from each fraction were cultured in 2 ml of 0.3% agar in McCoy's media with 10% fetal calf serum, 2.5 micrograms transferrin, 1 microgram insulin, and 1% penicillin-streptomycin. Colony growth was assessed after 14 days of culture at 37 degrees C and 6% CO2. Tumor colony growth was seen in eight of 36 (22%) patients with histologically negative marrows as well as in four of seven (57%) patients with known involvement. Mean colony growth per 2.5 X 10(5) cells for all 12 patients was 4.3 colonies for F X 1; 8.8 for F X 2; and 2.7 for F X 3. In contrast mean growth from the F X FD was 1.0 colonies. Cells with clonogenic potential could be demonstrated from F X 2 and F X 3 in seven of 12 and eight of 12 patients, respectively; in F X FD four of 12 patients had tumor growth. We conclude that separation of marrow samples by DDG concentrates malignant cells with clonogenic potential at least 8-fold compared to FD separation and that the sensitivity of the clonogenic assay in detecting marrow involvement by SCLC is enhanced by DDG sedimentation.