Prolactin (PRL) is a polypeptide hormone of the pituitary origin, that expresses over 300 separate biological activities, including its involvement in the regulation of immune functions. The hormone's immune capacities are related, among others, to comitogenic activity, prevention of immune cell apoptosis, stimulation of interleukins and antibodies production. Prolactin acts as a potent positive modulator of immunity to some protozoan parasites. It is well established that the hormone stimulates IFN-gamma and many other TH1-type cytokines production during Toxoplasma gondii, Leishmania sp. and Acanthamoeba castellanii infections. Recent studies suggest that human prolactin may be a regulator of antiparasitic activity against Plasmodium falciparum. On the other hand pregnancy-associated hyperprolactinemia may have a relevant contribution to reactivation of latent infections caused by many helminthic parasites, like Ancylostoma sp. or Necator sp. It is possibly connected with the process of transmammary transmission of hookworm infection to breast-fed newborns. Moreover, an increase in endogenous circulating prolactin during late pregnancy and lactation in ewes infected with Haemonchus contortus, promotes the phenomenon of periparturient egg rise. High prolactin levels have also been seen in dairy cattle suffering from other trichostrongylids infections. In this article we have discussed the role of prolactin as an important regulator of immunity to parasites.