Three chief cell types were studied in 8 canine mixed mammary tumors--luminal epithelial cells, myoepithelial cells, and filament cells, the last being the "stellate" cells recognized by light microscopists. Most cytoplasmic filaments in the myoepithelial cells were 5 to 8 nm thick, whereas another type of filaments were scattered, 8 to 10 nm thick. Almost all cytoplasmic filaments in the filament cells were 8 to 10 nm thick. In the adenomatous parts of the tumors, myoepithelial cells were chiefly seen, along with luminal epithelial cells. Filament cells were rarely observed without intercellular accumulation of mucoid. The filament cells were largely confined to the myxomatous and chondromatous parts of the tumors. The presence of cells of transitional forms in adenomatous areas suggested that the filament cells were derived from the myoepithelial cells.