A new method was developed to define the distribution of mast cells in normal human skin. The population density was determined in a number of zones of different depth in the dermis. Two regions of the same limb were studied systematically. Both were shown to have mast cell populations of similar size and distribution, with a maximum density immediately below the dermo-epidermal junction, gradually falling to a minimum density in the deeper layers of the dermis. Small secondary peaks were less clearly defined at the base of the dermis. These findings should provide baseline data for comparisons with mast cell population size and distribution in different areas of normal skin, or in similar areas in clinical disorders or experimental conditions.