The in vivo immunizing potency of diphtheria toxoid and formalin-treated corss-reacting material (CRM197, a nontoxic mutant protein) was compared in guinea pigs. Major antigenic differences between the two untreated proteins were also tested in rats. The results showed that diphtheria toxoid and CRM197 were equally effective immunogens, but only if the latter was treated with formalin in the same concentration (0.7% vol/vol) was that of the toxoid. Formalin treatment rendered the antigens more resistant to enzymatic proteolysis by trypsin in vitro.