Abstract: A unique clonal cell line, designated MMQ, is disclosed. The MMQ cell line is characterized by the presence of functional dopamine receptors and the secretion of only the prolactin hormone. The MMQ cell line is derived from the pituitary tumor 7315a, the tumor being characterized by the secretion of both prolactin and adrenocorticotrophin and an absence of functional dopamine receptors. The cell line also contains functional somatostatin and vasoactive intestinal polypeptide receptors. The cell line is prepared by dispersing the 7315a pituitary tumor cells into a population of single cells and incubating the cells for a period of time in plastic flasks containing incubation medium. Two distinct populations of cells are formed, one adhering to the plastic flask and the other floating within the plastic flasks. The floating cells are removed and cloned.