In a cross-over study in 12 normal individuals, the pharmacokinetic parameters of cefalotin, cefradine and cefazolin were determined after intravenous injection of 1,000 mg of each substance. The microbiological activities in urine and serum were determined using the agar diffusion test; the pharmacokinetic data were calculated by a computer system on the basis of a Fortran programme. Cefazolin has significantly higher serum concentrations than the other two cephalosporins, distinctly longer serum half-lives, higher protein binding, and smaller apparent volumina of distribution. In 36 inpatients with mainly chronical and acute infections of the urinary tract, we tested the antibacterial effectivity, the compatibility, and the application modalities of cefazolin. In 29 patients we had a satisfactory clinical result, in 25 cases we achieved the elimination of bacteria by the end of the therapy. The compatibility of cefazolin was good; apart from a minor, reversible, liver-specific increase in enzymes in 6 patients, no side effects could be detected.