The mBin1b is a beta-defensin gene identified in the mouse epididymis. In the current report, its expression pattern and antibacterial activities were characterized, and a transgenic (TG) mouse model was developed in which mBin1b was exclusively overexpressed by up to 50-fold over normal levels in the caput epididymis. The experimental animals are healthy with normal reproductive activity, but are more resistant to epididymal infection from Escherichia coli than normal animals. The expression of IL1a and IL1b in the epididymis was decreased in the TG mice, which suggests that mBin1b has a role in the regulation of inflammatory response in the epididymis.