SummaryHuman α‐lactalbumin (α‐LA), a milk protein normally produced by, and restricted to, functionally differentiated breast epithelial cells has been shown immunohistochemically to be a good marker for breast cancer. Antibodies with specificity only for human α‐LA have not previously been reported. The present study documents the production and characterization of an IgG1murine monoclonal antibody with specificity restricted only to human α‐LA. This monoclonal antibody, designated ET‐1, was purified by affinity chromatography using human α‐LA. Specificity of ET‐1 for human α‐LA was established by enzyme linked immunosorbent assay (ELISA), absorption studies, immunoprecipitation and by immunoblotting. ET‐1 binds to human α‐LA with a KDof 7·41 × 10‐8mol/l and does not cross‐react with bovine α‐LA. This unique reactivity of ET‐1, which is not inherently shared by polyclonal antisera, should enable the antibody to be used for the development of a sensitive immunoassay for circulating human α‐LA.