Thirteen of 280 (4.6%) blood cultures collected over a 12-day period were positive for Clostridium sordellii, a spore-forming anaerobe, rarely considered a human pathogen. Nosocomial bacteremia and intrinsic contamination of material used to culture blood were excluded as the source of the organism. Contaminated tincture of thimerosal used to swab the rubber stoppers of blood culture bottles prior to venting (aerobic) or during blind subculturing after 24 hours of incubation (anaerobic) in the clinical microbiology laboratory was determined to be the cause of the pseudobacteremia. After appropriate safe-guards were implemented, we have continued to use tincture of thimerosal for these procedures with no further growth of C sordellii from blood cultures. The importance of less-conspicuous steps in the routine processing of culture material have been reemphasized.