An enzyme-linked immunosorbent assay was developed to detect rabbit immunoglobulin G antibodies against purified lipopolysaccharide of the rough Re mutant of Salmonella minnesota. The time necessary for each step was investigated, and this resulted in a test that could be completed in 1 working day. Serial dilutions of rabbit sera drawn throughout immunization with the Re mutant revealed an elimination of the prozone effect upon prolonged immunization. We interpret this to demonstrate an increase in immunoglobulin G affinity for the lipopolysaccharide on prolonged immunization. We propose that the enzyme-linked immunosorbent assay prozone effect be investigated for use as a measure of antibody affinity.