1. A comparison of several published methods for analysing plasma protein turnover data has been undertaken, with particular reference to rapidly metabolized proteins such as members of the complement series. 2. With the exception of the equilibrium time method, most methods proved adequate for the determination of overall fractional catabolic rates. A further exception is that the renal clearance method becomes invalid when the fractional catabolic rate approaches the renal iodide clearance, but this may be the method of choice for slowly metabolized proteins if accurate urine collections can be ensured. 3. For the measurement of the ratio of extra-to intra-vascular protein pool size a clear preference emerged for the method of C. M. E. Matthews (1957, Physics in Medicine and Biology , 2 , 36–53). For rapidly metabolized proteins the calculations must be preceded by a correction for the non-protein bound iodide retained in the intra- and extra-vascular spaces. 4. The accurate calculation of fractional catabolic rate in the extravascular pool generally requires more experimental data than are commonly collected as well as an accurate correction for non-protein bound label that remains unexcreted. Only two techniques hold promise of accurate results: Nosslin's rate equations method and a development of Vitek's deconvolution method described herein. Nosslin's integrated rate equations method is particularly affected by systematic errors in renal iodide clearance estimates and should probably not be further used for this purpose.