There are a lot of leather artifacts out there that were overly saturated with leather “dressings” in the second half of the 20th century. These dressings are mostly mixtures of fats and oils, applied to keep the leather flexible and looking “cared for.” Too much of this material can congest the fibers and prevent the leather from keeping its balance of moisture content and its proper pH value. As the dressing ages and breaks down it moves back to the surface as “spew,” disfiguring the artifact. The leather also can shrink and distort under these conditions, problems that cannot be reversed. If the doctors’ bags have good form, as Barbara said, leave well enough alone. There are a few ways to clean them without doing damage and a few surface treatments to either retard powdering and flaking or to provide a better looking and more repellant finish, but these should be determined and applied by an experienced objects conservator.