Abstract: The hormone calcitonin promotes fertilizing ability in mammalian sperm and is useful for treatment of conditions of low fertility in humans and animals. Human, porcine or salmon calcitonin may be formulated in a pharmaceutical composition comprising a pharmaceutically acceptable carrier for topical application, e.g. as a cream or jelly containing up to 100 ng/ml or more of salmon calcitonin or up to 2 &mgr;g/ml or more of human calcitonin. Calcitonin may also be administered parenterally, orally, or nasally. For improving in vitro fertilization or artificial insemination methods calcitonin is added to sperm prior to use e.g. salmon calcitonin present at a concentration of from 5 to 50 ng/ml or human calcitonin at a concentration of from 20 to 200 ng/ml in the sperm preparation. Calcitonin levels in seminal plasma can also be used to diagnose infertility.