A highly efficient method employing NaSCN as a chaotropic agent was used to dissociate the membrane-bound porphobilinogen-deaminase. The same sequence of steps was applied for purifying both soluble and membrane dissociated porphobilinogen-deam inase. The chromatographic behaviour of both proteins was quite similar. Euglena gracilis deam inase appears to exist in an equilibrium mixture of two active species of relative molecular masses of 40000 and 20000.