Although previous studies have demonstrated an association between interferon-gamma (IFN-γ) promoter genotype and antibody response kinetics in chickens, the protein levels that may mediate such a gene–trait association have not been determined. The objective of this study, therefore, was to determine the correlation of circulating IFN-γ levels with both the IFN-γ promoter polymorphisms and antibody response in order to evaluate the potential role of IFN-γ protein in mediating genetic control of antibody response in chickens. Antibody response after Salmonella enteritidis (SE) vaccination at day 10, antibody response to sheep red blood cells (SRBCs) and killed Brucella abortus after immunizations at 19 wk and 22 wk, and serum IFN-γ protein level were measured in an F₂ population derived from inbred lines. A single nucleotide polymorphism in the IFN-γ promoter region was associated with IFN-γ protein expression as measured by an enzyme-linked immunosorbent assay after both primary and secondary immunizations. Higher IFN-γ protein level was correlated with higher antibody level to SE and with increased maximum level and decreased time to reach the maximum secondary antibody response to SRBCs. These results suggest that one of the mechanisms by which promoter polymorphism of IFN-γ affects antibody production in chickens may involve the circulating level of IFN-γ protein.