The pharmacokinetics of aspoxicillin [2S,5R,6R)-6-[(2R)-2-[(2R)-2-amino-3-(methylcarbamoyl)propionam ido]-2- (p-hydroxyphenyl)acetamido]penicillanic acid) in 10 subjects with normal kidney function and in 20 patients suffering from impaired renal function were examined after an i.v. short-term infusion of 4 g for a period of 20 min. In contrast to available semi-synthetic penicillins, aspoxicillin shows a slightly longer half-life elimination. As the substance is mainly excreted renally, the areas under the curve (AUC) are larger in cases of impaired renal function. Mathematical correlations can be established between the AUC and the renal function parameters creatinine and glomerular filtration rate. Dosage reduction factors are then derived which allow appropriate dosages to be established for the substances under examination. Dosages for differing degrees of impaired renal function are given in tables. Since sufficiently high and long-lasting urine levels are achieved, it is reasonable to use aspoxicillin as treatment of urinary tract infections in patients suffering from end-stage renal failure.