Microbiological, cytological, histopathological, and immunohistochemical investigations were carried out on four dairy cows affected by Serratia marcescens mastitis. The animals under study were from a herd of 120 lactating cows bred in the province of Rome. In the above herd, S. marcescens mastitis showed a prevalence of 20.8%. S. marcescens was the only bacterial agent isolated, prior to and after slaughter, from the teat milk, the mammary gland and the supramammary lymph nodes of the four cows under study. Cytologically, the four subjects exhibited high cell counts in their milk, with an average of up to 5,570,000 cells/ml in S.marcescens-infected quarters. Macroscopically, nodular lesions were apparent scattered throughout the mammary parenchyma, with enlargement of the regional lymph nodes. Histologically, a chronic, non-purulent mastitis, characterized by a marked fibrous tissue proliferation and the coexistence of corpora amylacea within the glandular alveoli, was observed in association with chronic hyperplastic lymphadenitis involving the supramammary lymph nodes of the four cows. Immunohistochemically, S. marcescens was demonstrated, by means of monoclonal antibodies, both in the mammary gland and in the supramammary lymph nodes from these four animals.