In this paper I analyze the tension between realism and antirealism at the basis of Kantian constructivism. This tension generates a conflictive account of the source of the validity of social norms. On the one hand, the claim to moral objectivity characteristic of Kantian moral theories makes the validity of norms depend on realist assumptions concerning the existence of shared fundamental interests among all rational human beings. I illustrate this claim through a comparison of the approaches of Rawls, Habermas and Scanlon. On the other hand, however, objections to moral realism motivate many Kantian constructivists to endorse the antirealist claim that reasonable agreement is the source of the validity of social norms. After analyzing the difficulties in the latter strategy, I try to show how a balance between the realist and antirealist elements of Kantian constructivism can be reached by drawing a sharper distinction between the justice and the legitimacy of social norms.