Abstract: The study was conducted to determine the immunomodulatory effects of Effective Microorganisms (EM®) on Horro and Fayoumi chickens at Agricultural Research Center and National Veterinary Institute (Debre Zeit). A total of 450 chickens (225 from each breed) were used in this study. Birds were grouped according to treatment: EM-treated (with feed, with water, with feed and water) and non-treated (only SRBC treated and Non-EM, Non-SRBC) controls. EM was given daily from the 3rd week of age for 5 weeks. Birds were injected with SRBC on the 4th and 6th week to see the total, IgG and IgM antibody responses. Antibody measurements were made using hemagglutination technique. The findings show that: (1) EM application has significantly increased antibody responses to SRBC, (2) there was no difference in antibody responses between the two breeds, or between the three modes of EM application. From these, it can be concluded that EM has a positive immunomodulatory effect when provided to chicks with feed or water. This study did not consider the cellular arm of the immune response and EM response to infections with specific pathogens was not investigated, collectively demanding further research in these and other issues if EM has to be used as good feed additive.