A synthetic peptide with enhanced pro-fertility action was produced by inclusion of additional amino acids at the carboxyl end of a previously disclosed synthetic peptide. Improvement in bioactivity over the previously disclosed peptide was demonstrated. A direct comparison of an earlier known synthetic peptide and an extended peptide involved brief exposure of sperm in vitro to one or the other peptide at several concentrations. When sperm then were evaluated in vitro using an egg-membrane substrate, an increased percentage of sperm bound for cells exposed to the new extended peptide. Similarly, when fertility of sperm after artificial insemination was the criterion, a greater percentage of eggs was fertilized by sperm exposed to the new extended peptide. In one preferred embodiment, this enhanced pro-fertility action was achieved with a peptide having a 68 amino acid sequence (SEQ ID NO 1:): Cys-Gln-Ser-Leu-Gln-Glu-Tyr-Leu-Ala-Glu-Gln-Asn-Gln-Arg-Gln-Leu-Glu-Ser-As n-Lys-Ile-Pro-Glu-Val-Asp-Leu-Ala-Agr-Val-Val-Ala-Pro-Phe-Met-Ser-Asn-Ile-P ro-Leu-Leu-Leu-Tyr-Pro-Gln-Asp-Arg-Pro-Arg-Ser-Gln-Pro-Gln-Pro-Lys-Ala-Asn- Glu-Asp-Val-Cys-Val-Asn-His-His-His-His-His-His.