Hormonally defined serum-free media have been developed for growth and functional studies with kidney epithelial cell cultures. Not only can several established kidney tubule epithelial cell lines (MDCK and LLC-PK₁) be grown in a serum-free environment (1,2), but in addition primary cultures of kidney tubule epithelial cells can also be grown serum-free (1–4). Investigations with primary kidney cell cultures are particularly advantageous for several reasons. First of all, kidney cells can be grown in vitro from the animal of choice. Thus the results of tissue-culture studies can be more closely correlated with animal studies. Second, new tissue-culture systems can be developed that more closely resemble the kidney cells in vivo than presently available established kidney cell lines.