Sea urchin egg proteins extracted with KC1 are mostly TCA-soluble and, conversely, those extracted with TCA are KCl-soluble. Both groups are water-insoluble and show fluctuations in --SH content during the division cycle. The fluctuation of the --SH groups of the KC1soluble protein of the whole egg is due to a --SH ~ --S--S--interchange within the freely reacting groups and not within the sluggish and masked --SH groups of the protein. The --SH content of the KCl-soluble protein of the egg cortex also fluctuates in a similar way.