A formula has been derived for the calculation of renal clearance with the use of a single plasma sample. The formula is based on a one-compartment model. A small correction for non-immediate mixing and non-uniform distribution of the tracer was calculated from empirical data. The accuracy in the calculation method depends on how exactly the distribution volume is known and at what time the blood sample is taken. The expected standard deviation in the clearance value was calculated from data of mean value and spread for the distribution volume of 99Tcm-DTPA. In an investigation of 39 subjects with 99Tcm-DTPA, a standard deviation of 5 to 6 ml/min was obtained in comparison with a standard method for clearance calculation. This value is in good agreement with the expected one.