Feedstuffs and additives with immunomodulatory characteristics were investigated to reduce losses caused by disease and to optimize the production potential of the bird. Therefore, the objective of this study was to assess the influence of yeast extract and prebiotics on broilers fed a prestarter diet, and the impact of ambient temperature on maternal and postvaccinal humoral immune responses. The birds were fed diets supplemented with yeast extract, prebiotics, or both in the prestarter phase (1 to 7 d), and from d 8 on, all birds received the same diet. The birds were vaccinated against Newcastle disease virus and infectious bursal disease at 8 and 18 d of age. The addition of yeast extract and prebiotic in the prestarter phase did not change the humoral immune response against Newcastle disease virus or infectious bursal disease throughout the production cycle of the broilers. The higher temperature increased IgG levels, especially maternal antibody titers, despite the immunomodulating effect of heat stress until the second week of life. DESCRIPTION OF PROBLEMThe immune system of the newly hatched chick is prepared to fight pathogens in a nonspecific manner through innate immunity, and can develop specific modes of defense through cellular and humoral immunity, which requires contact with antigens [1]. Acquired immunity differs from the innate responses by possessing specificity in recognition of foreign invaders (antigens) and the development of memory; therefore, it results in a more rapid response than was elicited during the primary exposure when the bird is reexposed to an antigen [2].