Recently, a stimulation test with colony stimulation factors was found useful to predict bone marrow cellularity. We examined the usefulness of this test to predict the bone marrow involvement in patients with malignant lymphoma. A single dose of GM-CSF (5 mu g/kg) was given subcutaneously to 12 patients with malignant lymphoma. Six patients had normocellular bone marrow. Five of these 6 patients showed an increase in neutrophils of more than 5 x 10(9)/l after GM-CSF administration. The remaining 6 patients who had bone marrow lymphoma involvement had a neutrophil count of less than 5 x 10(9)/l. Therefore, the change in neutrophil count with a cut-off value of 5 x 10(9)/l distinguished the marrow lymphoma involvement with a sensitivity and specificity of 83% and 100%, respectively. The nadir neutrophil count was significantly higher in patients with normal marrow than in patients with marrow involvement after chemotherapy (P < 0.02). In conclusion, the GM-CSF stimulation test is useful to predict the marrow involvement in patients with malignant lymphoma.