Synopsis
The potency of an inactivated Newcastle disease vaccine containing a number of strains of virus (vaccine 1) was greater in 28‐day old‐chicks of a hybrid strain (hybrid A) than in Rhode Island Red X Light Sussex chicks. Vaccination with one PD50 of vaccine 1 or vaccine 2 (a single strain vaccine) produced only minor changes in the electro‐phoretically determined serum protein composition in two hybrid strains (A and B) at a time when maximum immunity had developed.