The immunosuppression caused by virulent and vaccine strains of haemorrhagic enteritis administered in turkey, is assessed, by their influence on the serological response to Newcastle disease vaccination. The histological modifications of lymphoid organs after haemorrhagic enteritis vaccination are also studied. It is concluded that the vaccine virus has an immunosuppressive effect comparable to those of the virulent virus, but the duration of this effect is very short with some variations depending on genetic breed of turkey.