In this study, we investigate the possible effects on the human movement kinematics of the presence of a humanoid robot during an interaction. We conducted an experiment in which 11 participants were required to grab a cube and to drop it in the hands of a robot, a human and in a rectangular box. Using this setup, we explore whether the kinematics of the participants' gestures would be particularly influenced by the presence of the robot and whether this influence would be due to the fact that the robot is considered as a possible social partner. The results show that the condition that includes the robot partner leads to kinematic modulation that are similar to the condition including the human partner. Furthermore, there are significant differences between the condition including the robot and the one with the box. Finally, our results show that the participants pro-social behavior is correlated with the perceived agency of the robot as evaluated by a user questionnaire.