The purpose of this paper is to describe positive externalities of operations of castle and palace museums in Poland for private companies in local tourism sector. The paper presents the results of the author's original research, which was based on the multiple case study method. Results show that castle and palace museums in Poland generate positive externalities for private business operations in local tourism sector in three forms, i.e.: location, view and image. They appear in production functions of firms in the shape of: a reduction of customer acquisition costs, a reduction of production costs due to economies of scale, a creative contribution to product, logo, name and brand. A model of the impact of the operations of castle and palace museums on the production function of local tourism companies through the mechanism of external effects has been named: Museum Radiation Model.