Our investigations of the mating reaction of Chlamydomonas revealed a surprisingly intricate series of interrelated events. Adhering sites are moved to the flagellar tips in a fashion highly reminiscent of the capping of surface ligands over the centriolar regions of lymphocytes (28). Tipping is prevented by the gam-1 mutation and by agents that interact with tubulin; the molecular mechanism(s) for the inhibition effects are currently being sought. Tip locking appears to be accompanied by the accumulation of a dense material beneath the tip membrane, a postulated alteration of axonemal structure, and an immobilization of component(s) involved in surface motility. Two mating signals are then transduced to the locked-in cells who respond by shedding cell walls, activating mating structures, and fusing together. Signal transmission and/or reception is sensitive to such agents as trypsin, chymotrypsin, and cold temperature. Once zygotic cell fusion has occurred, tip unlocking and a reversal of the tip activation response appear to occur in parallel. Since all of these events can occur within 30 sec, the mating reaction serves as an experimental paradigm for studying rapid cellular responses to specific membrane-membrane interactions.