We described before that the chromatographically purified "human plasma ultrafiltrate bioactive fraction" (humoral factor tentatively denoted as tumor basic protein--TBP) regulates in vitro release of ACTH from pituitary adenomas stimulating the hormone release from the tumors showing low hormonal activity in vitro and inhibiting ACTH production in vitro by highly hormonally active pituitary tumors. In this study we describe growth promoting effects (determined by 3H-TdR incorporation assay) of TBP (5 microg/l, i.e. 10% w/v plasma equivalent concentration) for 10 non-functioning pituitary tumors. The effects of TBP appeared to negatively correlate with the in vitro growth abilities of the tumors that were otherwise dependent on the duration of the clinical symptoms of the tumor presence. Hence, similar to its effects on hormonal activity of the pituitary tumors, TBP stimulated the growth of the tumors which did not express high spontaneous 3H-TdR intensity, but did not stimulate the cells with high capacity of spontaneous 3H-TdR incorporation. Moreover, all the tumors that were stimulated by TBP were nononcocytic adenomas while oncocytoma cells were not stimulated at all. Thus, TBP shows activity of humoral (plasma) factor involved in the growth regulation of pituitary adenomas that might be used to define the growth abilities of these tumors, especially in case of null cell adenomas and oncocytomas as were the tumors used in this study.