One hundred and fifty chicks were used to study the effect of E.necatrix infection on the immune response of Newcastle disease vaccine. The chick en were classified into 7 experimental groups and 3 control groups. They were vaccinated at age of 30 days of age. with komarov vaccine. They were infected with 2000 sporulated E.necatrix oocysts 3 days before vaccin ation in the first group. Similtaneously in the second group and after 3, 18, 30, 60 and 90 days after vaccination in the succeeding 5 groups. The serum haemagglutinating inhibition titre and the faecal oocysts output of the different group < strong>s were determined. The comparison between the results of the experimental groups and those of the control groups showed that E.Necatrix infection of the vaccinated chicks resulted in suppression of the immune response to the vaccine.