centrifugation, adjusted to p H = 7.2 by dilute hydrochloric acid and placed on a column (10 cm in length, 4.5 cm in diameter) of DEAE-cellulose (DEAE-Sephadex A-25) whose chloride form has been washed with tris[(hydroxymethyl)amino]methane/HCl buffer (PH = 7.2) until the eluate has p H = 7.2. The column is eluted with buffer until no more porphobilinogen is removed from it. The eluates are evaporated in a vacuum to ca. 1800 mi, then adjusted to p H = 4 by dilute hydrochloric acid and treated with 20 % mercury(I1) acetate solution. The resulting precipitate is centrifuged off, washed with 1 % mercury@) acetate solution, and suspended in ca. 3 ml. of water. H2S is passed into the suspension to decompose the mercury compound. Then the precipitate is centrifuged off, nitrogen is passed into the liquid to remove H2S, and the p H of the liquid is adjusted to p H = 4 by dilute hydrochloric acid. This precipitates crude porphobilinogen which is centrifuged off after having been kept for 12 h in a refrigerator. Just sufficient 0.5 N ammonia is added to the centrifuge tube to dissolve the porphobilinogen, undissolved material is centrifuged off, the p H is adjusted carefully to 4 by glacial acetic acid for precipitation of the porphobilinogen, and the whole is left overnight in a refrigerator. The crystals are filtered off under suction and washed with ice-cold dilute acetic acid (PH = 4) and immediately thereafter with a little cold acetone. Drying in a vacuum desiccator gives 73 mg (12 %) of porphobilinogen.