Fragments of cerebellar cortex taken from adult rats were maintained for 7 days in organ culture. Inclusion in the medium of aprotinin, a polypeptide proteinase-inhibitor which also has carbohydrate-binding properties, was beneficial to the survival of neurons in the molecular layer, Purkinje cells and axons. This effect of aprotinin was mediated during the first 3 days in vitro. In a discussion of the various actions of aprotinin, it is concluded that the principal effect of the polypeptide on cultured central nervous tissue is to inhibit neuronal lysosomal proteinases activated by the trauma of explantation.