Are marginal notes marginal? Backed by a book from the private library of Emil von Behring the paper reflects whether annotations are peripheral sources or whether they can give some hints concerning biographical details. Behring's traces of reading are exposed and discussed by using the example of Otto Rot's Arzneimittel der heutigen Medicin (1877). The marginal notes demonstrate Behring's intensive work on hypnotics, sedativs, and analgetics which were possibly used for the therapy of others or himself. The findings will be compared to other sources of Behring's personal papers.