Neonatal poultry exhibit a transient susceptibility to infectious diseases during the first week of life that stems from inefficient host defense mechanisms. Yet, the initial host immune response to pathogens is a critical determinant of disease resistance and susceptibility. With this context in mind, novel ways to stimulate or modulate the hosts’ natural immune response is emerging as an important area of interest for food animal producers including the poultry industry. Specifically, we have been investigating new modulation strategies tailored around the selective stimulation of the host’s immune system, and particularly rapid acting innate immunity, as an alternative to direct targeting of microbial pathogens. One such approach that we have been investigating is the use of a group of cationic peptides produced by a Gram-positive soil bacterium, Brevibacillus texasporus (BT peptides). When provided as a feed additive, BT peptides significantly induced a concentration-dependent protection against cecal colonization and extraintestinal colonization by Salmonella enterica serovar Enteritidis (SE). This protection is not the result of direct antibacterial activity of the BT peptides on the SE since the concentrations used were below the minimum inhibitory concentration for SE. We also found that BT are not absorbed in the intestine, but still induce a significant up-regulation in the functional efficiency of peripheral blood heterophils and monocytes. The mechanisms of this immune modulation are unknown. Here, using in vitro models for measuring: (1) leukocyte oxidative burst, (2) changes in leukocyte cytokine and chemokines gene expression profiles, and (3) phosphorylation of the mitogen activated protein kinases (MAPKs) in leukocytes, we evaluated the role of BT peptides as priming mediators for heterophil and monocyte responses at the level of cell function, gene transcription/expression, and cell phosphorylation following stimulation with inflammatory agonists. BT peptides primed both heterophils and monocytes for an increased oxidative burst and up-regulation in transcription of the pro-inflammatory cytokines IL-1β and IL-6 and inflammatory chemokines CXCLi1 and CXCLi2 induced by inflammatory agonists. In addition, BT peptides induced a rapid (10 min) phosphorylation and activation of the extracellular signal-regulated kinase (ERK1/2) and p38 kinase pathways in primary chicken heterophils. Lastly, we assessed the effects of feeding BT peptides for the first 4 days post-hatch, on transcriptional changes on pro-inflammatory cytokines, inflammatory chemokines, and Toll-like receptors (TLR) in the ceca of broiler chickens. After BT feed removal, chickens were then challenged with SE and mucosal gene expression measured at 1 or 7 days post-infection by quantitative real-time polymerase chain reaction (qRT-PCR). Feeding BT peptides had no direct effect on TLR gene expression, the BT peptides primed cecal tissue for increased (P< 0.05) expression of TLR4, 15, and 21 upon infection with SE on days 1 and 7 post-infection (pi). Although feeding the BT peptides primed the cecal tissue for increased expression (2-5 fold) of pro-inflammatory cytokines (IL-1β, Il-6, IL-18) and inflammatory chemokines (CxCli1, CxCli2) in response to SE infection 1 day pi, we found that the BT peptide fed birds had a 5-16-fold increase in cytokine and chemokine expression 7 days pi with SE challenge when compared to the chickens fed the basal diet. Taken together, we conclude that BT peptides, acting through MAPK pathways, enhance leukocyte functional and pro-inflammatory cytokine and chemokine gene transcription activities. These small cationic peptides may prove useful as immune modulators in neonatal poultry.