This series represents nearly three decades of Johnson's correspondence with art dealers in Europe and America, as well as with experts and scholars, many of whom acted as liaison and critic to Johnson's acquisitions. Some of the most eminent names in art history are among Johnson's most prolific advisors; namely Bernard Berenson, W. R. (Wilhelm) Valentiner and R. Langton Douglas. A number of receipts and invoices filed along with the correspondence also document many of Johnson's purchases, including two receipts written, respectively, by the Danish artists P.S. Kroyer and V. Johansen to M. Alex Harrison. Harrison may have acted as Johnson's agent for the purchases of their paintings in 1887 and 1888. Paperwork regarding shipping and customs clearance are filed in the general alphabetical files.
Nearly all letters written by Johnson are photocopies, which the Museum's Johnson Collection curators compiled from other research repositories. (His correspondence to Valentiner is the one major exception.) Most of his letters to Berenson are from the Archivio Berenson at the Biblioteca Berenson, Harvard University Center for Italian Renaissance Studies at Villa I Tatti (Florence, Italy); one, dated Sept. 19, 1901, is from an unidentified repository. His letters to August Jaccaci were copied from the Archives of American Art, and almost all correspondence to and from Wilhelm Bode was supplied by the Central Archives of the National Museums in Berlin (Germany). These folders of photocopies are also filed alphabetically by name of correspondent.
At the end of the series is a group of reference folders consisting of material also prepared by the Johnson Collection curatorial staff. These papers are transcripts, summaries or photocopies of the preceding letters. The transcripts are particularly useful when trying to decipher Johnson's nearly illegible handwriting. Most of the reference material pertain to Johnson's correspondence with Berenson. There are also summaries of his correspondence with Roger Eliot Fry, one of the conservators Johnson engaged, and Herbert Horne, an art collector and historian of Italian Renaissance art. Also included as reference are photocopies of correspondence documenting Johnson's acquisitions on behalf of the committee that oversaw the fund Mrs. William P. Wilstach bequeathed to the City of Philadelphia.