Differential renal protein clearances were measured in twelve patients suffering from renal amylloidosis complicating familial mediterranean fever. Seven proteins examined included transferrin, IgG, IgA, IgM, C′3, ceruloplasmin and α2M. Eight out of twelve patients had non-selective proteinuria, three had moderately selective proteinuria, and one had highly selective proteinuria. A comparison was made between the estimation of selectivity by simple inspection of differential protein clearances and by three different methods involving measurement of the slope of the regression line between the logarithm of differential protein clearance and the logarithm of the molecular weight of the various proteins. It was concluded that none of these methods was superior to simple inspection of the differential protein clearance results, in assessing selectivity as an index of glomerular pore si