The determination of serologic titers to infectious organisms is a valuable tool for quantitating exposure to disease organisms. Raccoons (Procyon lotor) were live-trapped from September 1989 to October 1993 and samples collected from two distinct locations in west-central Illinois (USA); a state recreational facility (Park) and privately owned farming property (Farm). Sera were submitted for testing Leptospira interrogans (serovars bratislava, canicola, grippotyphosa, hardjo, icterohemmorhagiae, and pomona), canine distemper virus (CDV), pseudorabies virus (PV), and Toxoplasma gondii. Two-hundred and twenty-two (48%) of 459 raccoons were seropositive for L. interrogans. Eighty-five (23%) out of 368 raccoons were seropositive for canine distemper virus. Eighty-two (17%) of 479 raccoons raccoons were seropositive for pseudorabies virus. One hundred and eighty-four (49%) of 379 raccoons were seropositive for T. gondii. A significant difference (P < 0.05) in seroprevalence for L. interrogans between the park (43%) and farm (52%) areas was found. A correlation between increasing age and seroprevalence was found for L. interrogans, CDV, PV, and T. gondii. Furthermore, there was a significant difference in seroprevalence for T. gondii during the spring trapping seasons (73%), when compared with the fall (33%). This type of information on exposure to infectious agents is important for developing control programs to manage raccoon-human and raccoon-domestic animals interactions.