Tissue plasminogen activator (tPA), which is identical to alteplase, is a normal component of human colostrum and breastmilk.[1-3] Levels in milk are highest in colostrum and decrease rapidly during the first week, followed by a sower decrease over time.[2,3] Because alteplase is a large protein molecule with a molecular weight of about 59,000 Da, absorption is unlikely because it is probably destroyed in the infant's gastrointestinal tract. No information is available on the clinical use of alteplase during breastfeeding. Until more data become available, alteplase should be used with caution during breastfeeding, especially while nursing a newborn or preterm infant.