Introduction: Because of the increased use of im plantable access systems, the incidence of blood stream and catheter infections associated with these systems has concomitantly increased. Classically, he parin-lock flush solutions were used to prevent thrombosis; more recently, vancomycin was added to the solution to prevent infections caused by Gram- positive bacteria, particularly coagulase-negative Staphylococci. Disorders due to Gram-negative organ isms have now appeared in oncologic patients. We therefore tested the addition of colistin to heparin- vancomycin solutions. Colistin was chosen for its good activity against Gram-negative bacteria (98% susceptibility in our hospital), its good tolerance due to low systemic passage, and its low cost. Methods: We developed formulations contain ing heparin (100 IU/mL) and various concentrations of vancomycin (10-500 μg/mL) and colistin (10-100 μg/mL) in 0.9% NaCl. Each sterile solution was tested for physical and chemical compatibility (spectropho tometry, nuclear magnetic resonance, and pH mea surements) and its antibacterial activity (against ox acillin-resistant Staphylococcus aureus, Enterococcus faecium, Klebsiella pneumoniae exhibiting broad- spectrum betalactamase (BSBL), imipenem-resistant Pseudomonas aeruginosa) for 2 months at 4°C and at room temperature. Results: The most suitable combination of drugs is heparin (100 IU/mL), vancomycin (100 μg/mL), and colistin (100 μg/mL). This flush solution main tains activity when stored at 4°C for up to 1 month. Conclusions: We feel that the combination of heparin, vancomycin, and colistin can be used as a flush solution for indwelling catheters.