Abstract 1. 1. The actions of methyl methane sulphonate, isopropyl methane sulphonate and Tretamine on gametes of sea urchins have been examined. 2. 2. The rate of cleavage of eggs fertilized with sperm previously treated with methyl methane sulphonate was much slower than normal. Pretreatment of eggs with this drug produced qualitatively similar results. 3. 3. Treatment of sperm with isopropyl methane sulphonate at similar concentrations did not produce cleavage delays. There was also no evidence of a cleavage delay when unfertilized ova were first exposed to isopropyl methane sulphonate. 4. 4. Tretamine acted on ova at low concentrations to produce a marked increase of abnormalities which were probably due to polyspermic fertilizations.