Aim: Multidrug-resistant gram-negative isolates have become a problem in everyday clinical practice in intensive care units. Colistin was once used for treatment of gram-negative bacteria, but its systemic use has been completely abandoned due to nephrotoxicity. It was aimed to assess the in-vitro susceptibility of several gram-negative isolates to determine a standardized breakpoint for colistin. Methods: One hundred thirty-two multidrug-resistant nonfermentative gram-negative isolates were tested. Minimum inhibitory concentrations were determined using an agar dilution technique.