It is well known that firm integration can help resolve the problems caused by relation specificity. This paper shows that the same princinple can be extended to the relation-specific investments among employees. Using a simple cooperative-game theoretic model, it shows that economic agents invest more in each other when they are connected more directly with fewer contractual links between themselves. This result can be applied to explain why the employees of the same firm have more incentive to invest in long-term relationships with each other than with the employees of other firms.