The present report focuses on how the differences in the plasmin (PL) system between sheep and goats are reflected in their response to subclinical intramammary infection (IMI), particularly in respect to casein (CN) degradation and milk yield (MY). Plasmin is the major proteolytic enzyme in milk, where it is found mostly in its inactive zymogen – plasminogen (PLG), which is activated by plasmin activator (PA). Both plasmin activator (PA) and PL activities in goats in late lactation and goats with subclinical IMI are negatively correlated with MY and the coagulating properties of milk, suggesting that this system is important in goats as well. The higher CN content along with higher PL activity in sheep compared with goats resulted in higher CN degradation products that include factors that down-regulate milk secretion, which explains the more acute response in MY. Sheep are more vulnerable than goats to sub-clinical infections in terms of MY.