Differences in the immunogenicity of native and formalinized cross reacting material (CRM197) of diphtheria toxin in mice and guinea pigs and their implications on the development and control of diphtheria vaccine based on CRMs. 
Immunogenicity of native and formalinized cross reacting material (CRM197) of diphtheria toxin (DTx) was assessed in mice and guinea pigs.  For the primary response, mice produced similar levels of diphtheria toxoid (DTxd) IgG antibodies to both native and formalinized preparations of CRM197 though the antibody levels were significantly lower than those elicited by conventional DTxd (P < 0.05).  In contrast, guinea pigs showed significantly higher levels of DTxd IgG antibodies to the formalinized CRM197 preparation than the native preparation (P < 0.001) after single injection.  These differences in the immunogenicity of mice and guinea pigs to native and formalinized CRM197 preparations have implications for the development and control of diphtheria or other vaccines involving the use of CRMs.