Cefonicid concentration has been determined microbiologically in cortical and medullary tissue in 30 patient undergoing surgery because of neoplastic disease localized within the kidney. Each subject received 1 g of cefonicid intramuscularly in a single administration. The patients were divided into six groups, from which samples of blood and tissue were collected 2, 4, 6, 8, 12, 24 h respectively after treatment with the drug. The mean peak serum levels appeared at the second hour (74.96 +/- 8.14 mcg/ml) and the decay shows a monoexponential behaviour, reaching a minimum value of 5.4 +/- 2.33 mcg/ml at the 24th hour. In the cortical tissue of the kidney the peak levels appeared at the fourth hour (26.22 +/- 8.87 mcg/g), while at the 24th hour levels were about 3.08 +/- 0.81 mcg/g. A very similar behaviour could also be observed in the medullary tissue of the kidney with peak levels at the fourth hour (25.82 +/- 10.06 mcg/g) and levels of 3.48 +/- 0.85 mcg/g at the 24th hour. A delay in the decay of tissue levels in comparison with the decay of blood levels could be observed from the eighth hour.