Maternal sources of transforming growth factor-β1 (TGF-β1) are shown here to contribute to the normal appearance and perinatal survival of TGF-β1 null newborn mice. Labeled TGF-β1 crossed the placenta and was recovered intact from various tissues after oral administration to mouse pups. TGFβ-1 protein was also detected in cells recovered from breast milk. In immunohistochemical analyses, TGF-β1 null embryos and null newborn pups born to TGF-β1 heterozygotes stained positive for TGF-β1, whereas those born to a null female were negative and had severe cardiac abnormalities. These results suggest an important role for maternal sources of TGF-β1 during development and, more generally, provide evidence for maternal rescue of targeted gene disruption in the fetus.