Purpose: This research is situated in the context of breast cancer detection where the standard procedure is the succession of an initial mammography (MX) examination and a supplementary Ultrasound (US) scan. One major difficulty of this procedure results from the fact that breast geometry changes between both examinations due to different patient's positions. The proposed system facilitates this combined examination by keeping the breast geometry and by adding a US probe guidance robot to the mammography system. Methods: A co-manipulation system is set up where the robot and user simultaneously manipulate the probe towards the target previously localized in MX images. Calibration procedures and robot control are detailed. Results: A test protocol was presented to conduct two tests that are both related to the medical application. The first tests aims at evaluating robot guidance for localizing a lesion which was previously defined in the X-ray images. The second tests aims at quantifying robot influence when scanning a target lesion. The studied task consists of a pointing/scanning exercise, where the US beam intersects a breast lesion. Conclusions: The experiments show a significant increase in examination quality when using robot guidance as compared to the non-assisted examination.