Prior to fertilization in Campanularia flexuosa a capacitation-like interaction occurs between the spermatozoa and the epithelial cells that lead to and surround the eggs. Trypsin treatment of egg packets (eggs surrounded by epithelial cells) prevents the capacitation-like action and results in loss of fertilizability. Following treatment, the normal fuzzy surface coat of the epithelial cells is reduced from 200 A to 50 A. Even with increased amounts of sperm the fertilization rate of trypsin-treated eggs in the egg packets never reaches normal levels. This indicates loss of available epithelial cell surface interaction sites. Epithelial cell surface material is solubilized by incubation of egg packets in calcium- and magnesium-free sea water. Treatment of trypsinized egg packets with the solubilized material increases the fertilization rate. The possibility that the necessary capacitation-like interaction is mediated by the cell surface is discussed.