Bovine lactoferrin in involuting mammary tissue was identified by immunohistochemistry and tissue explant culture. Immunoreactive lactoferrin was associated with mammary epithelial cells. Immunostaining for lactoferrin increased during involution, in contrast to declining immunostaining of epithelia for the milk-specific protein β-lactoglobulin. Immunostaining for lactoferrin also was observed at the basal region of alveolar epithelia, perhaps in association with basement membrane components. Lactoferrin was preferentially synthesized in involuting mammary tissue compared with lactating tissue. Synthesis of lactoferrin in the involuting mammary gland occurs despite the apparent decline in synthesis of milk-specific proteins.