During a 17 month period, 25 hospitalized adult patients had blood cultures reported as positive for Pseudomonas maltophila. Review of the hospital records suggested that these were contaminants and that blood for coagulation studies and for cultures that were subsequently positive had been drawn simultaneously. The source of contamination appeared to be black-top evacuated collection tubes used for coagulation studies in adults. Cultures of the liquid anticoagulant tubes yielded a pure growth of greater than 10(5) colony forming units (CFU)/cc of Ps. maltophilia on blood agar. Mock trials demonstrated that following venipuncture by syringe, inoculation of contaminated black-top tubes prior to inoculation of blood culture bottles would yield false-positive blood cultures (pseudosepticemia). One patient being treated for streptococcal prosthetic valve endocarditis and having frequent coagulation studies with blood obtained via direct venipuncture into evacuated collection tubes was found to have superinfection of his prosthetic valve with Ps. maltophilia at autopsy. Prosthetic valve infection may have occurred after reflux of contaminated anticoagulant from an evacuated collection tube directly into the vein. Contaminated evacuated collection tubes are a potential source of confusion in the diagnosis of infection as well as a potential source of true infection.