The importance of inductive interaction betweeni tissues of the developing embryo has been aiimply demonstrated but the mechanisms remain obscure (1). Studies of inductive processes in the imiammalian eimbryo have been extremely limited due to technical difficulties. Recently it has been found that very early rudimiients of the mouse submiandibular salivary gland (2), kidney, and lung (3), continue surprisingly nor-mal morphogenesis at the glass-clot interface in Carrel flasks, and that each of these rudiments can be separated into viable epithelial and imesenehymal coini-ponents by exposure to trypsin-as has been described for chick rudiments by MIoscona and Moscona (4). These procedures have allowed study of epithelio-mesenchyimial interaction by observing the behavior in ivitro of heterogeneous combinations, eg, submandibular epithelium and kidney mesenchyme (nephrogenie cord), or kidney epithelium (ureteric bud) and submandibular capsular mesenchyme. The results of one such experiment are reported here to indicate that:(1) the method affords approaches to problems of inductive interaction of embryonic tissues;(2) as has been inferred on indirect evidence (5), inductive processes sii-milar to those described in lower vertebrates also occur in mammals;(3) even in the rela-tively simple epithelio-mesenchyinal systems considered here, at least two types of inductive processes appear to be operating.