The evident relationship between mast cells and tumors of various tissues and organs is a fascinating enigma. The association of mast cells with naturally occurring skin tumors, a subject which has not received much previous attention, was examined in the present study. Material and Methods Ten % aqueous formalin-fixed specimens of human skin which embraced both the tumor and the contiguous tissue (the term "contiguous tissue" as used in this paper refers to the dermal, but not the epidermal, tissue adjoining the tumors), the diagnosis having been made on sections stained with hematoxylin and eosin, were sectioned and treated with 0.01% toluidine blue.1 We have found aqueous formalin to be a serviceable fixative for mast cells in human skin despite objections which have sometimes been raised to its use.1 The types and numbers of tumors examined were as follows: The mast cells in each tumor and in the contiguous