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Abstract: The recognition and binding of a free-swimming spermatozoon to an ovulated oocyte is one of the most important cellular interactions in biology. While traditionally viewed as a simple lock and key mechanism, emerging evidence suggests that this event may require the concerted action of several sperm proteins. In this study we examine the hypothesis that the activity of such proteins may be coordinated by their assembly into multimeric recognition complexes on the sperm surface. Through the novel application of blue native polyacrylamide gel electrophoresis (BN-PAGE), we tender the first direct evidence that human spermatozoa do indeed express a number of high molecular weight protein complexes on their surface. Furthermore, we demonstrate that a subset of these complexes displays affinity for homologous zonae pellucidae. Proteomic analysis of two such complexes using electrospray ionization mass spectrometry identified several of the components of the multimeric 20S proteasome and chaperonin-containing TCP-1 (CCT) complexes. The latter complex was also shown to harbor at least one putative zona pellucida binding protein, ZPBP2. Consistent with a role in the mediation of sperm–zona pellucida interaction we demonstrated that antibodies directed against individual subunits of these complexes were able to inhibit sperm binding to zona-intact oocytes. Similarly, these results were able to be recapitulated using native sperm lysates, the zona affinity of which was dramatically reduced by antibody labeling of the complex receptors, or in the case of the 20S proteasome the ubiquitinated zonae ligands. Overall, the strategies employed in this study have provided novel, causal insights into the molecular mechanisms that govern sperm–egg interaction. [Copyright &y& Elsevier]