Gamete interaction triggers a variety of responses within the egg, collectively referred to as egg activation. In addition to the hallmarks of calcium release and fertilization envelope elevation, there are cytoskeletal rearrangements, protein tyrosine phosphorylation, and an increase in pH, among others. The ultimate goal of these concerted activation events is entry of the newly fertilized egg into the cell cycle. However, the molecular mechanisms which promote downstream cell activation events remain poorly understood. One model suggests that sperm deliver an "activating factor" upon fusion with the egg plasma membrane, while a second model proposes that the egg receptor for sperm transduces a signal that mediates a cascade of subsequent events. It also is possible that multiple pathways are activated. As a first step toward testing the hypothesis of receptor-mediated signal transduction, we have investigated the tyrosine phosphorylation state of the sea urchin egg receptor for sperm using specific antibodies. The present work indicates that the sperm receptor is phosphorylated by an egg cortical tyrosine kinase in response to sperm or purified ligand (bindin) binding. Maximal phosphorylation was reached within 20 sec. These data support the hypothesis that the sperm receptor is a gamete recognition protein which responds to ligand binding and focus attention on the question of the role of this tyrosine phosphorylation signal in egg activation.