McNickle acquired copies of anthropologist Sol Tax's correspondence from the library of University of Chicago. Tax is well-known for his contributions to anthropology, including his work on the Chicago Indian Conference in 1961. The correspondence, which is mainly between Tax and McNickle, covers topics such as Tax's involvement with AID (he was one of a group of directors), McNickle's solicitation for Tax's opinions on job candidates at the University of Saskatchewan, and correspondence about articles and books.