The invention relates to an active agent having contraceptive properties. Said active agent is a molecular fraction of venom from the Latrodectus mirabilis spider. The active agent is a peptide having sequence SEQ ID NO 1 or sequences at least 85% similar, obtained by means of chemical synthesis or recombinant DNA technology. In a preferred embodiment, the active agent having contraceptive properties is a peptide having 10 to 30 amino acids, preferably 20 amino acids. Said preferred embodiment is illustrated by the sequence SEQ ID NO 2 or sequences that are at least 85% similar, obtained by means of chemical synthesis or recombinant DNA technology. The invention likewise relates to a method for obtaining the contraceptive peptide, which includes the cloning of the nucleotide sequence in a microorganism.; The contraceptive pharmaceutical composition includes at least one peptide as per above and one or more pharmaceutically acceptable carriers. Lastly, the invention relates to the use of the peptide which can be used to prepare a drug suitable for use as a contraceptive agent and as a spermicidal agent.