Micronomicin (MCR) at a daily dose of 120 to 360 mg was administered to patients with severe infections who had hematopoietic disorders as underlying diseases. Efficacy and safety of the drug were evaluated. The underlying diseases in the 56 patients included in the evaluation of efficacy were acute myelocytic leukemia (24 cases), acute lymphocytic leukemia (8), acute promyelocytic leukemia (6), acute monomyelocytic leukemia (4), acute monocytic leukemia (1), erythroleukemia (1), chronic myelocytic leukemia-blastic crisis (4), malignant lymphoma (3), aplastic anemia (2), and others (3). The infections were septicemia in 9 patients, suspected septicemia in 48, respiratory tract infection in 7, and perianal abscess in 2. The clinical efficacy of MCR was 'excellent' in 12 patients, 'good' in 17, 'fair' in 7, 'poor' in 30 for an efficacy rate of 43.9%. The efficacy rate classified according to infections was 22.2% in septicemia, 56.3% in suspected septicemia. The organisms isolated from the patients with septicemia were Escherichia coli in 2, Klebsiella pneumoniae in 2, Pseudomonas aeruginosa in 1, alpha-Streptococcus in 1, Serratia marcescens in 1, and Acinetobacter sp. in 1. The efficacy rate was 15.4% in the 13 patients whose causative organisms were identified. The efficacy rate for patients who had failed to respond to prior antibiotic therapy was 43.9%. The efficacy rate in patients (34 cases) with an initial neutrophil count less than 100/microliter was 44.1%. Side effect which might have been caused by MCR was skin eruption in only one episode among 83 episodes those were evaluated for safety.