Mood contagion is an automatic mechanism that induces a congruent mood state by means of the observation of another person's emotional expression. In this paper, we address the question whether robot mood displayed during an imitation game can (a) be recognized by participants and (b) produce contagion effects. Robot mood was displayed by applying a generic framework for mood expression using body language. By modulating the set of available behavior parameters in this framework for controlling pose and motion dynamics, the gestures performed by the humanoid robot NAO were adjusted to display either a positive or negative mood. In the study performed, we varied both mood as well as task difficulty. Our results show that participants are able to differentiate between positive and negative robot mood. Moreover, self-reported mood matches the mood of the robot in the easy task condition. Additional evidence for mood contagion is provided by the fact that we were able to replicate an expected effect of negative mood on task performance: in the negative mood condition participants performed better on difficult tasks than in the positive mood condition, even though participants' self-reported mood did not match that of the robot.