The electron microscope was used in quantitatively assaying acrosome reactions of sea urchin sperm ( Arbacia punctulata and other species) after exposure to solutions of egg jelly. Several physiological parameters which affected the precision of the assay were investigated. Each experiment was repeated at least three times and a minimum of 100 sperm were counted for each experimental variable. Replicate counts of sperm on the same or different grids were within 99 percent confidence limits of the original count. The percentages of acrosome reactions induced by solutions of egg jelly were consistently about 80 percent if fresh sperm were used and proper conditions were maintained. Percentages of acrosome reactions in sperm exposed only to sea water were rarely more than 10 percent. The time period during which acrosome reactions were induced coincided with the period of sperm agglutination induced by egg jelly solutions. Although egg jelly-induced sperm agglutination occurred well in the pH range 6.5-9.0, only low percentages of acrosome reactions occurred at pH levels below 7.5. At pH 8.5 and above, acrosome reactions occurred spontaneously in sea water. Consistently high percentages of acrosome reactions could not be obtained when sperm and egg jelly solutions were mixed unless the calcium content of the sea water was raised to 20-30 ÂµM/ml. Sperm of species other than A. punctulata had different calcium requirements. The minimum concentration of egg jelly, measured in terms of Âµg fucose/ml, necessary to induce maximum levels of acrosome reactions in A. punctulata was 0.5 Âµg/ml.