OBJECTIVE To determine whether microorganisms are able to survive in a solution of 50 mg/ml of 5-fluorouracil (5-FU) and, therefore, whether there is a risk of vial contamination with multiple use. METHODS Ten common nosocomial pathogens were tested. Minimal inhibitory concentrations (MICs) of 5-FU were determined for each organism. Organisms were then inoculated into 1 ml of 5-FU (50 mg/ml) and, after timed periods of exposure, were plated onto blood agar and incubated at 37 degrees C. Plates were checked daily for the presence or absence of growth. RESULTS The MICs of Klebsiella pneumoniae and Pseudomonas cepacia were within one log unit of the 10 mg/ml concentration of 5-FU used by some clinics. After incubation in 5-FU for 1 h, all species survived; after 24 h of exposure, five of the 10 species-Staphylococcus aureus, Staphylococcus epidermidis, Enterococcus faecalis, K. pneumoniae, and Proteus mirabilis-were still viable. CONCLUSIONS 5-FU has limited bactericidal activity, and there is a risk of contamination if 5-FU vials are used in multiple dose fashion.