CD9 has been shown to be essential for sperm/oocyte fusion in mice, the only non-redundant role found for a member of the tetraspanin family. CD9 can act in cis, reconstituting sperm/oocyte fusion when ectopically expressed in oocytes from CD9 null mice, or in trans, inhibiting sperm fusion when the large extracellular domain (LED) is added to CD9-positive oocytes as a soluble protein. In contrast to cis inhibition, the structural requirements of the trans inhibition by soluble CD9 LED are unknown. Here we show that human CD9 LED is as potent an inhibitor as mouse CD9 LED in mouse sperm/oocyte fusion assays and that CD9 LED can also inhibit sperm/oocyte binding. The two disulphide bridges that define membership of the tetraspanin family are critical for structure and function of human CD9 LED and mutation of a pentapeptide sequence in the hypervariable region further defines the critical region for trans inhibition.