© 2016 Elsevier Inc.We conducted a large-scale lost letter experiment in Beijing, a megacity with > 21 million residents, to test if the observed altruistic attribute of the letter recipient would induce more passersby to return the lost letters. The treatment letters were addressed to a nationally renowned charitable organization in China, while the control letters were intended to an invented individual. A total of 832 ready-to-be-posted letters were distributed in 208 communities across eight districts in the city. The overall return rate was 13%. Yet, the return rate of the treatment letters (17%) was nearly twice as high as that of the control letters (9%). The finding adds large-scale field experiment evidence in support of the other-regarding preferences theory. In addition, we also found that the lost letters were more likely to be returned if they were dropped in communities with a relatively higher income or a postal box located closer.