As horse owners, we often use hormones to modify what we consider "poor behavior" in our animals, most commonly with altrenogest (as in Regu-Mate®) or medroxyprogesterone acetate (Depo-Provera®). In humans, it is well known that synthetic progestins suppress the immune system, leading to an increased risk of diseases like HIV and herpes. But do these hormones have similar effects in our horses? We need to find out.