This study was undertaken to investigate the presence of mammosomatotrophs (MS) cells in the suckling mink. Using the double immunolabeling procedure, with colloidal gold as label, we demonstrated the existence of MS cells in these animals. Only one type of MS cells has been observed. These cells showed a great morphological similarity to classic prolactin (PRL) cells. MS cells of suckling mink were pleomorphic in appearance with many processes, their nuclei were irregular and their Golgi apparatus and endoplasmic reticulum were poorly developed. Their secretory granules were small (about 144 nm in mean diameter) and round. Two types of secretory granules have been found: monohormonal including PRL (the more frequent) and growth hormone (GH) (very scanty) granules, and bihormonal granules distributed between the former. We propose that MS cells of the mink, like other species, could represent an intermediate cell type in the transformation process of GH cells into PRL cells.