Chapter 2 presented a short discussion of the occurrence and routes of immune transmission and a summary in Table 1. Chapter 4 dealt with the intestinal absorption of macromolecules. The present chapter is a short review of systems that function in extraintestinal tissues in transmitting antibody and other macromolecules from mother to young. The bulk of the relevant information is derived from investigations carried out in cow udder, rabbit visceral yolk sac and human placenta. The latter two have been reviewed several times (see References 151, 154, 1214, 1264a, 1265, 1564, 1570, 1572, and 1573).