Using a three-stage game model, this paper analyzes the optimal level of anti-trust enforcement against joint production in relation to the degree of R&D spillovers. It shows that the optimal level of anti-trust enforcement is negatively related to the magnitude of R&D spillovers. In particular, the government should show some tolerance of joint production cartels only when R&D spillovers are sufficient for firms to cooperate voluntarily in their R&D activities. When R&D spillovers are so weak that firms do not cooperate voluntarily in the R&D stage, the government should intensify anti-trust enforcement against joint production to a prohibitive level.