The steady-state renal clearance of cefsulodin was studied in 6 patients with cystic fibrosis and 8 healthy controls. The drug was administered by constant rate infusion to obtain 2 values of plasma concentration, 2 and 30 mg/L. As an estimate of the glomerular filtration rate, the renal clearance of inulin was measured simultaneously.The results showed the figures for inulin clearance to be approximately 30% higher in cystic fibrosis patients than in healthy controls at both concentrations, and a corresponding increase in the renal clearance of cefsulodin was seen in patients over controls. The ratio between the renal clearances of the 2 substances was on average 0.9 in both groups.The correlation found between the 2 renal clearances (r = 0.75; p < 0.001) indicates that glomerular filtration rate has considerable influence on the renal elimination of cefsulodin. This finding emphasises the importance of glomerular filtration rate for the renal clearance of drugs in cystic fibrosis.