In three successive litters of a Duroc and Slovak White crossbred sow there was a 7.75% overall incidence of melanoma. Immunohistochemical labelling confirmed the mesenchymal origin of the neoplastic cells, indicated the absence of CD3 lymphocytes in the tumour, and proved the tumorous origin of surrounding non-pigmented cells. An abundance of macrophages (epithelioid cells) with increased endoplasmic reticulum was observed by electron microscopy, but large granular lymphocyte-like cells were few in number.