Ceftriaxone (CTRX), a new long acting antibiotic in the 3rd generation cephem group, was administered intravenously once or twice a day in daily doses of 1-6 g for at least 3 days to 86 patients with severe infections complicating hematopoietic disorders. Underlying diseases were acute leukemia in 41 cases, chronic leukemia in 3 cases, malignant lymphoma in 19 cases, myeloma in 7 cases and others. Most patients (55 cases) suffered from sepsis or suspected sepsis. As for efficacy rates classified by underlying diseases, the treatment was effective in 61.0% of patients with acute leukemia. As for efficacy rates classified by infections, the treatment was effective in 60.0% of patients with sepsis. No side effects were noted except rash in 2 patients. Abnormal hepatic functions were recognized in 3 patients but were not attributed to the agent in any case. The results indicate that CTRX is a safe and useful antibiotic for the treatment of severe infections accompanied by hematopoietic disorders.