We present here a systematic study on the purification of the diphtheria toxoid (Dtxd) produced at the Instituto Butantan, by adding only one step on the entire, process of its production. Aliquots of 1.0 ml of Dtxd were added to an equal amount of Q-Sepharose. previously equilibrated with 500mM Tris, pH 5.0-9.0 (increments of 0.5 pH units). The best condition for the Dtxd monomer adsorption was achieved at pH 9.0. The best condition for desorption was obtained with 300 mM NaCl. After studying the gel binding capacity for Dtxd, a column (C20/20) equilibrated with 500 mM Tris, pH 9.0, was prepared. The purification factor for Dtxd was 1.5. The final recovery of Dtxd was 68.75%, with 90.31% purity. The process methodology presented here is a very realistic sequence of separation steps, which is perfectly compatible with the production requirements. Vaccination with "toxoid highly purified toxin" is known to confer a strong immunity on people in the absence of undesirable reactions, which led experts of European Pharmacopoeia to recommend its use both for children and adult vaccination. (C) 2003 Elsevier Inc. All rights reserved.