The present report is an analysis of results of assays of diphtheria toxoids carried out over a four-year period at the Laboratory of Hygiene, Department of National Health and Welfare, Ottawa, Canada. A single preparation, a crude monovalent diphtheria toxoid, was used as a reference standard for all types of preparation (purified, crude, or adsorbed). The results suggest that the use of such a standard is justified even though there is evidence to show that adsorbed and plain toxoids, and toxoids of different grades of purity, do not have identical immunization curves. The merits of the present International Standard for Diphtheria Toxoid, Plain, for the assay of all types of toxoid are discussed. The results suggest that this is not particularly suitable for adsorbed preparations, probably because of its flatter immunization curve. For the latter preparations, the proposed international standard for diphtheria toxoid, adsorbed, should prove more practical and useful.