Two cases of systemic aspergillosis are described in dairy sheep from a flock in which fungal mastitis appeared subsequent to the antibiotic treatment of animals before parturition. Lesions characterized by necrosis and a pyogranulomatous exudate were observed in the mammary glands, supramammary and mediastinal lymph nodes, kidneys, lung, liver, heart, forestomachs and brain. The intense vasculitis with thrombosis observed in various organs, but especially in the mammary glands, suggested a haematogenous dissemination of the infection from this organ. The aetiological diagnosis was accomplished by the immunohistochemical staining of the fungal structures seen in the histological sections by the specific Aspergillus monoclonal antibody Mab-WF-AF-1 together with the isolation of Aspergillus fumigatus in pure culture from affected tissues.