The slow-cycling cells in the bulge of the outer root sheath may represent stem cells for the hair follicle. With each new anagen (growing) phase bulge cells would give rise to a population of transient-amplifying cells which differentiate into outer root sheath and matrix keratinocytes. The lowermost part of the telogen (resting) follicle is composed of bulge cells lying in close proximity to the dermal papilla. In the mouse the first hair follicle growth is characterized by rapid follicular neogen-esis, but the second and third growth cycles follow the normal follicular growth pattern. We have examined activity of cells in the bulge at the onset of anagen (growing phase) in the third hair cycle in Sencar mice, by treating animals at the end of the second telogen with colchicine to localize mitotic activity in the hair follicle. Mitoses were only seen in bulge cells during early anagen. This confirms that they proliferate transiently, solely at the onset of anagen and strongly supports the suggestion that bulge cells are the origin of the whole lower follicle in anagen.