A colorimetric assay based on the reduction of a tetrazolium salt [2,3-bis[2-methyloxy-4-nitro-5-sulfophenyl]-2H-tetrazolium-5-carboxanilide (XTT)] for rapidly determining the susceptibility of Pseudomonas aeruginosa isolates to bactericidal antibiotics is described. There was excellent agreement between the tobramycin and ofloxacin MICs determined after 5 h using the XTT assay and after 18 h using conventional methods. The data suggests that an XTT-based assay could provide a useful method for rapidly determining the susceptibility of P. aeruginosa to bactericidal antibiotics.