The antibody response to the O antigens of the patients' own gram-negative bacteria causing bloodstream infection in patients with malignancies was investigated using the hemagglutination test. Of the 87 patients, 42 (48%) produced specific antibodies in significant titers. Differences were noted between the various microorganisms. Of 44 patients with E. coli bacteremia 26 (59%) had an antibody response, whereas only 5 (33%) out of 15 subjects with Pseudomonas aeruginosa infection did. The antibody response of patients with solid tumors (61%) was significantly better than that of subjects with RES malignancies (30%). Documentation of the humoral immune response of patients to isolates from blood cultures excludes laboratory contamination and supports the relevance of the isolated microorganisms. Thus, for diagnostic purposes documentation of the immunoglobulin response supplements bacteriologic findings.