We investigated if complement component 1 inhibitor-like (C1-INH-L) protein found on human spermatozoa exists on mouse spermatozoa and is relevant to reproduction. We used Western blot analysis and immunofluorescence assays to detect C1-INH on murine sperm and tested the effects of the antibodies to C1-INH and albumin (negative control) on in vitro mouse sperm motility and fertilization. C1-INH-L, with molecular weight similar to human C1-INH (100 kDa), was present on the surface of spermatozoan head and midpiece. Treating mouse sperm with anti-C1-INH reduced the mouse sperm motility (P < 0.01), in vitro fertilization (P < 0.01) and embryo development rates (P < 0.01). Anti-albumin failed to do so. We conclude that C1-inhibitor-like protein is present on mouse sperm surface and appears to be relevant to reproduction.