In response to antitrust cases challenging the exclusionary conduct of dominant firms, some dominant firms offer an “appropriability defense.” This defense is the claim that prohibiting the challenged conduct would lessen the dominant firm’s return to investment in research and development (R&D), undermine that firm’s incentive to innovate, and harm the prospects for industry innovation. An appropriability defense should be questioned, and often rejected, if the dominant firm would be expected to increase its own R&D effort in response to increased R&D by its rivals after liability on a dominant firm is imposed. An analytical framework for determining whether a dominant firm would behave this way is provided, based on evaluating the firm’s likely incremental gain from new product development if its rivals also introduce new products relative to its gain if its rivals do not upgrade their products, and potentially observable factors relevant to making this assessment are identified. The application of the framework in individual cases is illustrated using the facts of three classic antitrust monopolization cases involving new product development: the Microsoft case involving Netscape and Java, the IBM plug compatibility cases (treated as a single case), and the FTC’s patent portfolio case against Xerox.