The therapeutic effectiveness of (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), cephazolin, cephacetrile and cephalothin was compared in the test model of estradiol-induced E. coli pyelonephritis in the rat. Cefazedone and cephazolin were similar in effect. The relations between results of treatment and microbiological and pharmacokinetic characteristics of the cephalosporins tested are discussed.