Cells in the secretion of involuting and non‐lactating mammary glands of the ewe were studied. Light microscopy studies revealed that the majority of the cells were mononucleated, but it was impossible to distinguish macrophages from sloughed epithelial cells. Electron microscopy studies showed that the majority of die cells were macrophages and that they phagocytosed polystyrene latex spheres. Very few epithelial cells were found and they were distinguished from macrophages by numerous microfilaments and blunt microvilli. Epithelial cells could he further distinguished from macrophages by the presence of tight junctions. Tissue culture of cells in secretion of mammary glands revealed the predominant cell type to be the macrophage. After 3 days in culture these macrophages formed multinucleated giant cells which contained acid‐phosphatase granules. Fluorescein‐labelled antiserum prepared against the cell cultures, stained cells in the lumens of alveoli and duets, but not epithelial cells, in frozen sections of mammary tissue. The study confirms previous findings that the large majority of cells from mammary glands in late involution are macrophages. It also points to the usefulness of non‐lactating mammary glands as a source of sheep and cattle macrophages.