Inclusion Criteria:  Hospitalized with an underlying diagnosis of hematological cancer or receipt of hematopoietic stem cell transplantation. Age ≥18 years. Absolute neutrophil count (ANC) of <0.5x10^9/L or <1.0x10^9/L with a predicted decline to <0.5x10^9/L within the next 2 days. Oral temperature of ≥ 38.3 °C once, ≥ 38.0 °C lasting for at least 1 hour, or ≥ 38.0 °C twice within 12 hours.  Exclusion Criteria:  Allergy to cephalosporins. Expected survival of ≤72 hours. Expected ANC recovery within ≤72 hours. Receipt of any antipseudomonal carbapenem therapy within the preceding 14 days. Documented current or past infection or colonization with ceftolozane-tazobactam- resistant Enterobacterales or P. aeruginosa. Pregnancy. Previous enrollment in CEF-10 Study (subjects previously enrolled in this study cannot be enrolled again during subsequent episodes of neutropenic fever). Ongoing therapy or planned continuation of therapy outside the National Center for Cancer Care and Research.