Muroctasin, a derivative of MDP, is known to augment the number of WBC via colony-stimulating factor. Muroctasin has been expected to be promising for application to leukopenia caused by anticancer chemotherapy. When WBC decreased to less than or equal to 3,000/mm3 after the 1st course of chemotherapy, 131 patients with lung cancer, who were previously classified by chemotherapy combination, were enrolled in the study and randomized into 3 groups, 200 micrograms (H), 100 micrograms (L) and untreated control (C) groups. The patients were then subcutaneously treated once daily for 6 consecutive days. WBC and its differential count were measured on days 4, 7 and 15 after commencement of the study. WBCs in H and L groups showed greater recovery than in C group. In WBC differential count, the recovery of neutrophil was prominent in muroctasin-treated groups. A portion of immature neutrophil in bone marrow was also increased by muroctasin treatment. In the present study, the usefulness of muroctasin in leukopenia was indicated when administered at dosages of 200 micrograms for 6 days.