It has been recently established that serum from human volunteers immunized with E. coli J5 vaccine prevents death of patients with gram-negative shock. The present study addressed the question whether the prophylactic administration of a similar amount of J5 antiserum could protect neutropenic patients from acquiring gram-negative infections. One hundred patients, the majority of which had acute non-lymphoblastic (63%) and lymphoblastic (29%) leukemia, presented 109 episodes of neutropenia. Sixty of the 100 patients underwent bone marrow transplantation. All patients were given one unit of either pre-immune (control) or J5 antiserum serum from volunteers at the onset of neutropenia. When compared to control serum, J5 antiserum given prophylactically did not reduce the number of febrile days, the number of gram-negative bacteremic episodes, or death from these infections. This inability to demonstrate a beneficial effect of prophylaxis with a single unit of J5 antiserum in prolonged neutropenia may have several explanations that are discussed.