Immunization of chickens with whole bacteria results in the production of antibodies specific to lipopolysaccharide (LPS), a major constituent of the outer membrane of Gram-negative bacteria. However, there is relatively limited information available concerning immune response of purified LPS in this species. In the present study, immune responses were examined in serum and egg yolk from two groups of chickens injected with entire LPS from Escherichia coli and lipid A free LPS from Salmonella typhimurium. The results demonstrated that the increase of antibody activity occurs first in serum, and then in egg yolk with a lag in time of 1 to 3 wk in both groups of chickens. However, the time of elevated levels of antibodies activity was much shorter in chickens immunized with S. typhimurium LPS (< 1 wk) than in those immunized with E. coli LPS (4 wk). A lack of lipid A is the S. typhimurium antigen may be a factor related to this difference.