1. Agglutination occurs in solutions of thyroxine in sea-water. This agglutination is permanent, and occurs especially, but not only, around the crystals which are present in solutions of thyroxine. It was observed in the sperm of species of Echinus, Solaster, Asterias and Buccinum.
2. There is chemotaxis of the sperm towards these crystals.
3. The resemblance of this type of agglutination to hetero-agglutination is discussed.