Primate cognition requires interaction processing.  Interactions can reveal otherwise hidden properties of intentional agents, such as thoughts and feelings, and of inanimate objects, such as mass and material.  Where and how interaction analyses are implemented in the brain, is unknown.  Using whole-brain fMRI in macaque monkeys, we discovered a network centered in medial and ventrolateral prefrontal cortex, engaged in social interaction analysis exclusively.  Exclusivity of specialization was found for no other function anywhere in the brain.  Two additional networks, a parieto-premotor and a temporal one, exhibited both social and physical interaction preference, which, in the temporal lobe, mapped onto a fine-grain pattern of object, body, and face selectivity.  Extent and location of a dedicated system for social interaction analysis suggest this function as an evolutionary forerunner of human mind-reading capabilities.