Objective: to develop a method for verifying ceftriaxone and determining its concentration in blood serum by means of capillary electrophoresis. Materials and methods. The study used ceftriaxone sodium salt (CJSC Pharmaceutical Company "Lekko"), model mixtures of biological fluids (whole blood, serum and blood plasma) with different concentrations of ceftriaxone; clinical biological blood samples taken during breast surgery against the background of perioperative antibiotic prophylaxis. The concentration of ceftriaxone was determined by using a Capel-105M capillary electrophoresis system (CJSC Lumex, St. Petersburg, Russia). Results. An express method has been developed for the determination of ceftriaxone in blood serum (sample volume - 0.1 ml), which involves the use of a precipitant of protein substances - crystalline ammonium sulfate, extraction of the organic system chloroform - isobutanol in a ratio of 3: 1, re-extraction of the substance into a working electrolyte diluted with water, and subsequent electrophoresis on the device "Capel-105M". Method validation indicates its linearity, correctness, precision and specificity; the lower limit of quantitation is 0.1 µg/0.1 ml biological medium. The effectiveness of the technique was confirmed in a clinical trial by determining the concentration of ceftriaxone in the blood serum 30 minutes after intravenous administration of 1 g of the drug for the purpose of perioperative antibiotic prophylaxis. Conclusion. The method for the quantitative determination of ceftriaxone in blood serum, performed by using a capillary electrophoresis system (“Kapel-105M”), meets the established validation criteria. At the same time, the identification of the antibiotic is carried out under selective conditions and requires a minimum amount of the object, thereby reducing the amount of co-extractive substances in the samples, which, in combination with the method of stacking during electrophoresis, provides a high sensitivity of the technique. It can be recommended for analysis in clinical diagnostic laboratories.