The artificial capillary culture technique provides a pericellular microenvironment that closely resembles the in vivo state. This may allow normal cells to maintain their differentiated functions for prolonged periods of time in vitro. Cells are maintained in a physiologic state without many of the artifactual conditions imposed by more standard methods of tissue culture. The technique is therefore especially well suited for the culture of cells of endocrine origin. Response of the cells to various physiologic agents are being studied as well as the secretion of various hormones over long periods with the possible application to continuous production of hormones for laboratory and clinical uses. Attempts to establish human cell lines that secrete significant quantities of growth hormone or insulin have been unsuccessful. While several cultures of a human pituitary adenoma have continued to secrete 2 micron g prolactin per day for a period in excess of 6 months, there is neither evidence of increasing prolactin nor indications that these cells have become established. BIBLIOGRAPHIC REFERENCES: Knazek, R.A. and Skyler, J.S.: Secretion of Human Prolactin in vitro. Proc. Soc. Exp. Biol. Med. 151:561-564, 1976. Skyler, J.S., Lovenberg, W., Rogol, A.D. and Knazek, R.A.: Characterization of Growth Hormone and Prolactin Produced by Human Tumors in Culture. Endocrinology 100: 283-291, 1977.