1. When sea-urchin eggs are fertilized an acid, the fertilization acid, diffuses out of them into the surrounding sea water. A claim has recently been made that the fertilization acid may be lactic acid, which is oxidized shortly after its production. 2. There is no significant difference between the amounts of lactic acid produced by unfertilized and just-fertilized eggs of Echinus esculentus. 3. The amount of lactic acid produced by fertilized eggs of this species is too small to explain the evolution of CO2 observed when the eggs are fertilized in manometer vessels. 4. The sea water round just-fertilized eggs was found to contain markedly less bicarbonate than the sea water round unfertilized eggs, confirming that an acid diffuses out of the eggs after fertilization. 5. There is some evidence that the small quantities of lactic acid produced by unfertilized and just-fertilized eggs of this species do not diffuse into the surrounding sea water. 6. It is concluded that the fertilization acid is not lactic acid.