Letters, notes, programs, photographs, and printed materials. The collection is comprised primarily of handwritten correspondence between Reiner and notable music figures including Samuel Barber, Leonard Bernstein, Aaron Copland, Lukas Foss, Darius Milhaud, Arthur Nikisch. Arnold Schoenberg, Richard Strauss, Igor Stravinsky, and Leo Weiner. Also of note, letters from writer and conductor Gian Francesco Milpiero and his wife Auna detailing wartime conditions in Italy (1946).

The collection contains approximately 1300 scores, half of which bear markings entered by Mikhashoff or his colleagues in preparation for performance. The collection includes manuscripts and print publications, both originals and reproductions.