1.0 g cefamandol was injected in 25 patients with various degrees of renal function. Half-life and distribution volume of this antibiotic and the glomerular filtration rate (GFR) were then measured. The distribution volume was 18.5 +/- 3.5% of body weight, similar to the extracellular space. Half-life and GFR were related in a way which could be expressed in the formula: half-life = 400 sqaure root GFR. Levels of blood concentration of cefamandol could be estimated from half-life and GFR, with the construction of "isoconcentration dosages", by which it is possible to achieve levels similar to those in normal subjects receiving a normal dosage.