One of the most fruitful concepts that arose from modern approaches to in vitro animal cell culture is that of cellular endocrinology. Historically, cellular endocrinology arose from attempts to develop functionally differentiated cultured cell lines, particularly those derived from the endocrine system (35). Many of the lines developed were responsive to or produced classicallydefined hormones, even though they were derived from tumor tissue that in some sense might be considered less differentiated than the normal tissue from which the