108 serum samples in 27 patients were collected after the first, before the second, after and before the fifth dose of 2 g every 24 h and assayed for ceftriaxone by microbiological method and high pressure liquid chromatography (HPLC). The statistical analysis of the results show a correlation coefficient equal to 0.95. The protein binding doesn't affect the microbiological method. The two methods are convenient for clinical use.