The level of antitoxin has been estimated in sera from lambs vac­cinated with bivalent lamb dysentry and pulpy kidney vaccine, in vivo by mouse test and in vitro by indirect haemagglutination test. The results showed a good correlation between the toxin neutralization and indirect haemagglutination tests. Studying the immune response of blackleg vac­cine in calves and guinea-pigs both by challenge and indirect huemugglu­tination test revealed that the test may be useful as an aid for the eval­uation of the protective potency of blackleg vaccine.