PMK stands for Pyro - Metol -Kodalk (Kodak's name for sodium metaborate). It is available in either a powder or liquid mix from Photographer's Formulary: http://www.photoformulary.com/index.htm. It has minimal effect on film speed. Grain is moderate to fine. It provides a beautiful tonal scale, especially in the highlights, which are almost impossible to block. The pyro stains the gelatin, so the negs looks greenish-yellow and need about a stop more exposure to print than ordinary negs. The developer is extremely easy and economical to use, with a very long shelf life. I use it exclusively, with Delta 100 & 400.(posted 9023 days ago)http://www.ravenvision.com/rvapeter.htm