Background: One of the major factors that may have negatively affected the results of many oral sensitization studies in animals has been unscheduled dietary preexposure of the test animals or their parental generations to the antigen under investigation. Objective: The influence of dietary preexposure to soy protein on oral sensitization studies with soy protein in Brown Norway rats was investigated. Methods: Brown Norway rats bred on a soy protein-containing diet for several generations (routine bred [RB] animals) were placed on a soy protein-free diet during and for at least 6 months before breeding (F0 group). Four generations of offspring were bred on a soy protein-free diet (F1, F2, F3, and F4 groups). RB and F4 animals were exposed to soy protein either ad libitum through drinking water or parenterally with an adjuvant. Results: In the F0 and F1 animals soy protein-specific IgG antibodies were still detectable, whereas no soy protein-specific IgG was detectable in the other generations tested. In RB animals no significant increase in soy protein-specific IgG titers occurred after exposure to soy protein. Enteral exposure of the F4 animals to soy protein resulted in sensitization to soy protein, with increased soy protein-specific IgG titers. Conclusions: These studies demonstrate that there is a continued expression of anti-soy protein antibodies in rats bred and raised on a soy protein-free diet for one generation. Not only must the test animals be bred and raised on a specified antigen-free diet, but their parental generations must also be bred in the same manner to avoid any problems in oral sen-sitization studies. Copyright © 1998 by Mosby, Inc.