We have carried out a study to evaluate the interference by cephalosporins with the measurement of creatinine by desk-top analyzers. The cephalosporins evaluated at concentrations of 0-250 mg/l were cefazolin sodium, cefoxitin sodium, cefotaxime sodium, and ceftazidime pentahydrate. The instruments evaluated were DT60 (Kodak, Rochester, USA), Seralyzer (Ames Division, Miles Laboratories, IN, USA), and Vision (Abbott Labs, Chicago, USA). All studies were done in plasma. None of the cephalosporins showed any interference with the DT60 analyzer. With the Vision and Seralyzer no interference was seen with cefotaxime or cefazolin. With cefazolin an increase of 10-20 mumol/l creatinine was seen for every 20 mg/l of drug; with cefoxitin there was an increase of 50-80 mumol/l of creatinine for every 100 mg/l of drug. Erroneous creatinine values may be found in patients taking cefazolin and cefoxitin and may lead to inappropriate clinical management.