The new cephalosporin derivative (6R,7R)-7-(2-[3,5-dichloro-4-oxo-1(4H)-pyridyl]-acetamido)-3-([(5-methyl-1,3,4-thiadiazol-2-yl)-thio]methyl)-8-oxo-5-thia-1-azabicyclo[4,2,0]oct-2-ene-2-carboxylic acid (cefazedone, Refosporen) has been tested for antibacterial activity in the infection and therapy model of acute E. coli pyelonephritis in rats. The reference substance was cephalothin. The tests showed a superiority of cefazedone which, however, could not be clearly confirmed by the significance test. Owing to its favourable pharmacokinetics in combination with good antibacterial activity, cefazedone can be added to the cephalosporins considered for use in clinical practice, especially so as the statistics for 1977 show that the causative agents of pyelonephritis which these drugs can often combat effectively are present in the majority of the urine strains: E. coli accounted for 45%, enterococci for 18%, and Proteus for 15% out of a total number of 6100 urine strains.