Fertilin is a heterodimer of alpha and beta subunits, both of which are members of the ADAM (A Disintegrin and A Metalloprotease domain)/MDC (Metalloprotease-Disintegrin-Cysteine-rich) family of proteins. We have previously demonstrated that recombinant forms of the putative extracellular domains of mouse fertilin alpha and fertilin beta bind to mouse eggs and inhibit sperm-egg membrane binding. In this study, we examined the roles of the disintegrin domains of fertilins alpha and beta by producing recombinant forms of fertilins alpha and beta that included the disintegrin domains (alphaDCE and betaDCE) or that were truncated so that they lack the disintegrin domains (alphaCE and betaCE) and tested the abilities of these proteins to bind to eggs and to inhibit sperm-egg binding. Fertilin betaDCE was able to inhibit sperm-egg binding, but fertilin betaCE was relatively ineffective, indicating that the disintegrin domain of fertilin beta is required for interactions with egg binding sites and/or for proper protein folding. Fertilins alphaDCE and alphaCE both inhibited sperm-egg interactions, but fertilin alphaDCE tended to be more effective. Thus, the presence of the disintegrin domain in fertilin alphaDCE apparently enhanced the ability of this recombinant protein to inhibit sperm-egg binding, either by interacting with egg binding sites or by improving the efficiency of protein folding. These data also indicate that the other domains of the fertilin alpha extracellular region (cysteine-rich and/or epidermal growth factor-like repeat) have the ability to block sperm binding and suggest that these domains of fertilin alpha may participate in sperm-egg adhesion.