In membrane remodeling during spermoocyte membrane fusion. Epididymal albumin may also be responsible for assembling the protein complex recognized by mAb TRA 54. In addition, the epididymal albumin-containing protein complex may be absorbed by the sperm acrosome, as occurs with other epididymal secreted proteins. Although the high molecular mass complex is not required to trigger the acrosomal reaction, this complex is gradually dispersed and fully exocytosed during the reaction. This forward movement of proteins could involve albumin as a carrier molecule. In rats, PES proteins are released from the epididymis in a testosterone-dependent manner, can bind to the sperm membrane when these cells pass through the epididymal lumen and may participate in sperm binding to the zona pellucida. We hypothesize that the same pattern of synthesis and function might apply to the complex recognized by mAb TRA 54 and that, in Staurosporine addition to epididymal PES, epididymal albumin may have an important role in fertilization after exocytosis of the albumin-containing complex during the acrosomal reaction. In agreement with this suggestion, the in vitro fertilization experiments described here showed that the addition of mAb TRA 54 to the fertilization medium significantly inhibited the fertilization rate of zona pellucida-intact oocytes but did not affect this rate in zona pellucida-free oocytes. Together, these findings indicate that the epididymal high molecular mass complex containing albumin is involved in the optimization of zona pellucida penetration by an acrosome-reacting sperm. In addition, epididymal albumin could facilitate the transportation and coupling of other molecules and enzymes that become tightly bound to the sperm surface and are required for the secondary binding of sperm to the oocyte membrane. In conclusion, we have identified a high molecular mass complex containing albumin in homogenates of caput epididymis. In addition, albumin mRNA was detected in testes and epididymis. Functional assays involving fertilization in vitro demonstrated that this high molecular mass complex has a role in fertilization. Since infertility in mice and humans is strongly associated with a lack of epididymal molecules, and since the high molecular mass complex containing albumin characterized here is also expressed by human epididymis, identification of the other molecules present in the epididymal complex recognized by mAb TRA 54 could be helpful in understanding male infertility. Numerous efforts were done to reduce the incidence and mortality of GSQCLC.