Employing the techniques of immunofluorescence, radioimmunoprecipitation, and radioimmunodiffusion, the development of antibody responses to RSV and BSA and the localization of immunoglobulin containing cells in the mammary glands, was studied in groups of pregnant rabbits after intravenous (IV), per oral (PO) or trans-tracheal (IT) immunization with RSV and BSA during late gestation. A predictable IgM and IgG and no IgA antibody response to RSV was observed in the serum. The secretory response to RSV was characterized by the regular appearance of IgA antibody in the colostrum and milk after IT and PO immunization but not after IV immunization. The proportion of IgA staining cells in the mammary tissues was found to be 50% higher in the animals immunized by IT or PO routes than by the IV route. No infectious virus or RSV antigen was detected in the breast. Most animals elicited IgG anti-BSA response in the serum, colostrum and milk, and a few evidenced IgA in serum. However, IgA response to BSA was notably absent in colostrum and milk. These observations suggest independent contributions of bronchus associated and gut associated lymphoid tissue to the development of mammary immunity. More importantly, significant differences exist between soluble dietary proteins and particulate viral antigens in their ability to induce breast milk antibodies.