Izumo, a sperm membrane protein, is essential for gamete fusion in the mouse. It has an Immunoglobulin (Ig) domain and an N-terminal domain for which neither the functions nor homologous sequences are known. In the present work we identified three novel proteins showing an N-terminal domain with significant homology to the N-terminal domain of Izumo. We named this region “Izumo domain,” and the novel proteins “Izumo 2,” “Izumo 3,” and “Izumo 4,” retaining “Izumo 1” for the first described member of the family. Izumo 1–3 are transmembrane proteins expressed specifically in the testis, and Izumo 4 is a soluble protein expressed in the testis and in other tissues. Electrophoresis under mildly denaturing conditions, followed by Western blot analysis, showed that Izumo 1, 3, and 4 formed protein complexes on sperm, Izumo 1 forming several larger complexes and Izumo 3 and 4 forming a single larger complex. Studies using different recombinant Izumo constructs suggested the Izumo domain possesses the ability to form dimers, whereas the transmembrane domain or the cytoplasmic domain or both of Izumo 1 are required for the formation of multimers of higher order. Co-immunoprecipitation studies showed the presence of other sperm proteins associated with Izumo 1, suggesting Izumo 1 forms a multiprotein membrane complex. Our results raise the possibility that Izumo 1 might be involved in organizing or stabilizing a multiprotein complex essential for the function of the membrane fusion machinery. Mol. Reprod. Dev. 76: 1188–1199, 2009. © 2009 Wiley-Liss, Inc.