The authors studied the inactivating action of dies on alexin. To do this research they used 50 dies of the different chemical series, and arrived at the conclusion that only 11 of these dies have that action: the ponceau, Congo red, methyl violet, auramin, eosin, Bengal rose, 'ecth' green, Paris vio¬let, methyl blue, pironin yellowish and blue anilin. It was verified that they always inactivate in constant proportion: 0,5 ml. of a solution at 1% and that this action seems to be independent of the physical constants of the dye (ele¬ctric charge, mollecular weight, maximum absorption in the espectrometer) and the intrinsic peculiarities of their chemical constitutions. The authors also verified that these dyes in their majority inactivated the fourth component termo-stable (C4) of the alexin, with exception of the Congo red and the methyl violet which inactivated the third component termo-estable (C3). The pironin yellowish showed to act on the 3rd and 4th components (C3 and C4) or over any indispensable element to their action.