Vaccine failures cause economic losses in the poultry industry. The hypothesis of this randomized, blinded, double controlled study was that 1 or more dosages of Gan Lian Yu Ping Feng (GLYPF; modified Yu Ping Feng San) would have immune-enhancing effects. Serum antibody, interferon gamma (IFN-γ) and interleukin 4 (IL-4) levels and immune organ indexes were evaluated at 7, 14, 28 and 42 days after infectious laryngotracheitis (ILT) virus vaccination. Chickens that were 45-days-old (336 total) were divided into 8 groups of 42 chickens each. In Groups I-III and IV-VI, 0.25 g/ml, 0.5 g/ml or 1 g/ml respectively of GLYPF were added to the drinking water for 3 days, beginning before or after vaccination respectively. Group VII received 0.0004 g/ml Wen Du Qing (positive control). Group VIII (negative control) was vaccinated, but received no herbal medicine. The serum antibody titers of Groups III and VI (GLYPF 1 g/ml) were significantly higher (p<0.01), than all other groups at 14, 28 and 42 days. Serum IFN-γ levels were significantly higher (p<0.05) and serum IL-4 levels significantly lower (p<0.05) in Groups III and VI compared to all other groups at 42 days. The organ indexes of Groups III and VI were significantly higher than Group VIII negative control. In conclusion, 1 g/ml GLYPF, administered in the drinking water 1 day before and 2 days after or 3 days after vaccination, can improve immune responses to ILT vaccination in chickens and may be useful to reduce vaccine failures on poultry farms.