Macrophages in bovine milk.

The high vacuolated mononuclear cells in milk from normal bovine mammary glands, usually designated epithelial cells, were examined for properties characteristic of macrophages. In vitro, these cells concentrated neutral red into cytoplasmic vacuoles, adhered rapidly to and spread on glass, and phagocytosed viable Streptococcus agalactiae and Staphylococcus aureus. Neutral red uptake was demonstrated in vivo, and the vacuolated mononuclear cells phagocytosed formalinkilled S aureus in vivo. These results suggest that these cells are macrophages.