Very little is known about antiprotease efects on cytodifferentiation and morphogenesis in mammals. The present study should shed some light on fundamental processes of differentiation. In the in vitro embryonic rat pancreas system chosen, antiproteases may be a probe for determining to what extent protease activity is required during morphogenesis and cytodifferentiation. This approach will be used as a screening technique for proteolytic antagonists which have differing substrate specificities and presumably sites of action.