Considerable changes have occurred during the 1980s in the clinical nature and diagnosis of bacteremia and fungemia in the immunocompromised patient. Cancer patients with prolonged neutropenia, many with indwelling catheters, and AIDS patients with both T-cell and B-cell deficiencies have changed the spectrum of organisms causing septicemia. There has been a shift to infection with gram-positive bacteria, including mycobacteria, and water-borne organisms, including Acinetobacter spp. and Pseudomonas spp. New blood culture systems, including a lysis-centrifugation system and radiometric methods utilizing resin broth media, remove antagonistic antimicrobial agents, and the lysis-centrifugation system routinely provides quantitation of organisms from the blood. Quantitation has been used to identify sources of infection, to differentiate contamination from true infection, and to monitor the course of antibiotic treatment.