A new method for the spectrophotometric determination of Evans Blue in plasma is presented. The method is based on the precipitation of the non-albumin fraction of the plasma proteins with polyethylene glycol, in order to eliminate the variable background absorption of plasma at the wavelength of maximum absorbance of Evans Blue. The accuracy and precision of the method is excellent: at an Evans Blue concentration in plasma of 5 mg . 1(-1) the coefficient of variation of the method is less than 1%. Five different procedures currently in use for the calculation of the plasma volume from the amount of indicator injected and the concentration at zero time (c0) were compared. Extrapolation to zero time of the early part (10 to 60 min) of the log concentration vs time curve turned out to the the best method for the calculation of c0, and hence yields the best estimate of the plasma volume.