Sea urchin spermatozoa provide an excellent model system for studying the signal transduction events underlying the acrosome reaction (1,2). Adults can be collected from tide pools, subtidally by SCUBA diving, or purchased from biological supply companies. When adults are injected with 0.5 M KCl, the semen comes out of five gonopores on the aboral surface and can be collected with a Pasteur pipet and stored on ice for up to 5 d. The most widely used species in cell research is the California purple sea urchin Strongylocentrotus purpuratus. Five milliliters of undiluted semen can be collected from a single male of 60-mL body volume. We have worked with as much as 100 mL of fresh semen for one membrane isolation. Each 1 μL of semen contains 4×10⁷ spermatozoa and equals 100 μg of total sperm protein. No other animal model provides such high numbers of spermatozoa and so much plasma membrane at such a low cost in time, labor, and money.