Chemotaxis of sperm toward an egg is a critical step in reproduction, particularly in aquatic environments, where sperm frequently travel long distances to contact an egg. Chemoattractants called sperm activating and attracting factors (SAAFs) were isolated from eggs of the ascidians Ciona intestinalis and Ascidia sydneiensis, and the structures of SAAFs were determined to be novel polyhydroxysterol sulfates, Ciinte-SAAF and Assydn-SAAF, respectively. However, the absolute configuration at C25 of the side chain remained unknown. For the complete structure elucidation of SAAFs, their two possible diastereomers with respect to C25 were synthesized. Ciinte-SAAF was synthesized starting from chenodeoxychoric acid. The steroid backbone was constructed via reductive 1,3-transposition of an allylic alcohol and the axial opening of an epoxide as key steps, and the side chain was introduced by Wittig olefination. Assydn-SAAF was synthesized starting from ergosterol. The steroid backbone was constructed by using an intramolecular pinacol coupling reaction and stereoselective reduction of a hydroxyketone as key steps, and the side chain was introduced by Julia-Kocienski olefination. Comparison of the NMR data of the synthetic specimens with those of the natural products, Ciinte-SAAF and Assydn-SAAF, led to the elucidation of the both absolute configurations at C25 as S. Thus, the complete structures were determined and the first syntheses of Ciinte-SAAF and Assydn-SAAF were achieved. Furthermore, structure-activity relationship studies of Ciinte-SAAF was carried out by using synthetic analog molecules, and molecular probes were prepared for elucidating the mode-of-action.