IgA and IgM antibodies were detected in rat milk after immunization with ferritin in Peyer's patches (Pp) 1 day after parturition but not after intramammary gland or intravenous immunization. The antibody levels decreased from day 9 to day 17 of the nursing period and were undetectable during a second lactation period. Despite the absence of milk IgM antibodies after intramammary gland or intravenous immunization, the serum levels of the IgM antibodies were similar after all three immunization methods. IgA antibodies were not found in serum after any of the immunization methods.IgG antibodies appeared in serum and milk after P. intramammary gland, and intravenous immunization. Milk and serum IgG antibodies from all the Pp-immunized animals decreased from day 9 to day 17 of the lactation period. After intramammary gland immunization, however, the IgG antibody levels increased in all the milk samples, but only in four of seven sera. The milk and serum IgG antibody levels were lower but still detectable during a second lactation period. Re-injection of ferritin in the Pp during a third lactation period resulted in higher levels of milk IgA, IgG and IgM antibodies than after the first injection. Rats with serum IgG antibodies against Escherichia coli 08 naturally present in their gut flora had no corresponding milk antibodies of any isotype. The results suggest tht milk antibodies of all three isotypes stem from local production in the mammary gland and that blood IgG and IgM antibodies originate at least partly from stimulation in Pp.