Laboratory conditions were established for the titration of diphtheria toxin and antitoxin in Vero Cell Cultures (CCT) and a comparison was made with the intradermal test (IDT) as currently used throughout the world. Working dilutions and cut off values were established by reading both the change in color of phenol red used as pH indicator and changes in cell viability in the microscope. CCT shows high reproducibility and higher detectability than IDT in the titration of both WHO Reference Standard and high titer horse antisera. In sera of guinea pigs immunized for potency testing of diphtheria toxoid the titre was approximately 10 times lower than in the IDT. The explanation is a subject of speculation. The Vero cell titration might be adopted as such for titration of diphtheria antitoxin. In the case of the toxoid potency test it could be used if the limit for the titration is adjusted to 0.2 IU considering the equivalence obtained between the two tests, by taking into account a ratio of 1:10 between CCT and IDT.