The nephrotoxicity of 1-[2-carboxy-8-oxo-7-(2-thienylacetamido)-5-thio-1-azabicyclo[4.2.0]oct-2-en-3- yl]methyl-pyridinium hydroxide (cephaloridine, Glaxoridin) in mice was investigated histopathologically in a series of 14 experiments. Using the PAS-reaction lesion formation consisting of epithelial desquamation in the tubuli contorti proximales, coarse cytoplasmic granulation of the tubuli recti proximales epithelium and hyaline cast formation was detected. Regeneration of the tubular epithelium followed subsequently. Optimal lesion formation was detected when mice were given two s.c. injections of an aqueous solution of the compound and the post mortem was carried out two days later. B5D2F1 hybrid mice were more sensitive than NMRI mice. However, a preexperimental acclimatisation period of at least ten days was necessary to obtain reproducible results.