The development of an immune response to rubella virus in milk, serum, and nasopharyngeal secretions was studied in lactating postpartum women after immunization with HPV-77 De5 or RA 27/3 live, attenuated rubella virus vaccine administered subcutaneously or intranasally. Over 69% of the women shed virus in milk after immunization. A predictable nasopharyngeal IgA and serum IgG antibody response to rubella virus was observed after subcutaneous or intranasal immunization with RA 27/3 vaccine. Little or no nasopharyngeal antibody response was seen after subcutaneous immunization with HPV-77 DE5 vaccine. A virus-specific IgA antibody response in milk was seen in all women. The presence of rubella virus in breast milk seemed to potentiate the peak levels of virus-specific antibody in the milk. Cellular immune reactivity to rubella virus in milk was observed in all vaccine groups. Thus, the virus-specific immune response induced in human milk after immunization with rubella virus vaccine may be intimately linked with the reactivity in bronchial lymphoid tissue.