We compute the capacity of mobile ad hoc networks(MANETs) when all the nodes in the network are endowed with M antennas. The derivation is based on a new communication scheme for wireless ad hoc networks utilizing the concept of cooperative many-to-many communication, called opportunistic cooperation, as opposed to the traditional approach that emphasizes on one-to-one communication. We demonstrate that the capacity of MANETs with multiple antennas is improved using cooperation as compared to non-cooperative schemes, i.e., point-to-point communication. Monte-Carlo simulation is used to validate the results.