The mean concentrations of antibiotics in the patients following the elective caesarean section were as follows: ceftriaxone--22.7 microg/I. vs cefazolin--44.8 microg/l. Six hours later, the concentration of antibiotic decreased, but the concentration of cefazolin was still over the MIC for sensitive bacteria. The mean concentrations of antibiotics following gynaecological surgery in the non-pregnant patients were as follows: ceftriaxone--12.0 microg/I vs cefazolin--30.1 microg/l. Six hours later, the concentration of antibiotic decreased.