Cottontail rabbits (Sylvilagus floridanus) usually are thought to succumb to infection with Francisella tularensis. Reports of a rabbit population from southern Illinois (USA) with a high prevalence of F. tularensis antibodies suggested that some cottontails survived infection with this typically fatal bacterium. Our goal was to examine the humoral response of cottontails from a study area in southern Illinois for which multiple serum samples existed. Multiple sera were collected from 79 cottontails from 1986 to 1990 and 63% gained, lost, or maintained ELISA titers of IgM and IgG isotype antibodies. The typical pattern of antibody response appeared to be IgM isotype antibodies first, followed by IgG isotype antibodies, with both generally increasing to high titers. Negative culture attempts of liver tissue from 51 cottontails with varying antibody responses suggested that chronic infection did not occur in rabbits that developed antibody. The significance of the cottontail antibody response in resolution or prevention of tularemia infection remains unclear.