Purified mitochondria from rat brain were incubated in vitro which microtubule-associated proteins (MAPs) that are known to bind specifically on sites present on the outer membrane. The bound molecules were detected by immunoelectron microscopy and the linear distribution of the label along mitochondrial profiles was analyzed by statistical methods. The results demonstrate that gold-conjugated antibodies are distributed in a non-random fashion on the surface of mitochondria, suggesting regional concentrations of MAPs-binding sites. This finding argue for the existence of specialized domains on mitochondria that are involved in the association of the organelles to microtubules in situ.