From the total material of 47 pituitary tumors, 42 had one cell line and five had two cell lines. The latter were found in tumors causing acromegaly either with secretion of growth hormone (GH) alone (n = 2) or in combination with prolactin (PRL) (n = 3). In tumors secreting only GH, biclonal cell lines occurred in four of 20 (20%) of the total number of cell lines. In tumors with concomitant secretion of GH and PRL, the corresponding proportion was six of 13 (46%). In only one case was it possible to identify two separate cell populations morphologically.