Importance: Highly pathogenic avian influenza (HPAI) A H5N1 has been recognized for nearly three decades as a threat to avian species and as a virus with pandemic potential if spillover into human populations occurs. Recently the virus has evolved capacity to infect many mammalian species, including dairy cattle, increasing the risk for human exposure and the pandemic threat. Sialic acids (SA) serve as binding sites for influenza viruses. The distribution of SA determines infectivity of specific influenza viruses across species and tissue tropism. Hemagglutinin (HA) of human and swine adapted influenza viruses bind primarily to SA with α2,6-galactose linkages and avian influenza viruses preferentially bind to SA with α2,3-galactose linkages. Recently, the bovine udder was found to contain SA with α2,3 linkages which allow the H5N1 virus to bind to bovine udder epithelium and to infect milk. The distribution of SA receptors in the human mammary gland is unknown. Objective: Evaluate normal human mammary tissue for the presence of both human and avian SA receptors. Design: Retrospective evaluation of lectin binding to sialic acids in human mammary tissue. Setting: US academic health center. Participants: Specimens obtained from de-identified women with residual non-malignant tissue following breast surgery at the University of California, San Diego. Exposures: Lectin affinity-staining of human tissue for the presence of SA. Main Outcomes and Measures: Presence or absence of lectin staining. Results: All mammary tissue samples demonstrated lectin staining for both SA receptors with α2,6-galactose and α2,3-galactose linkages. Conclusions and Relevance: The presence of SA receptors that can bind HPAI A H5N1 in human mammary tissue indicates that human milk could be infected during severe avian influenza infection. Pandemic preparedness must prioritize mechanisms to protect the safety of human milk.