The present study examined, using rats as a model, the effects of sex and age of exposure to dietary soya components on serum total and soya-specific antibody content. In Expt 1, Sprague–Dawley rats at 50 d of age were fed diets containing 20 % casein or 20 % alcohol-washed soya protein isolate (SPI) with or without supplemental isoflavones (ISF, 250 mg/kg diet) for 70, 190 or 310 d. The offspring were fed the same diets as their parents. In Expt 2, juvenile Sprague–Dawley rats at 30 d of age were fed diets containing 20 % casein with or without supplemental ISF (50 mg/kg diet) or increasing amounts of alcohol-washed SPI (5, 10 or 20 %) for 90 d. Exposure of rats to dietary SPI before the age of 28 d increased serum total IgA and IgM, and induced the production of SPI-specific IgA, IgG, IgM and IgE antibodies. Feeding of juvenile or adult rats with SPI elevated serum total IgA in females, while the opposite occurred in males, and markedly stimulated the production of SPI-specific IgM in females and IgG in males. Our data suggest that the effects of soya proteins and ISF on the production of serum total and SPI-specific antibodies appear to be sex dependent and also related to the age of exposure to soya in rats. However, the physiological significance of these immune responses remains to be determined.