A simple experimental technique was developed to provide an in vitro model for the study of human follicular keratinocytes. Anagen-phase human hairs were plucked from the scalp of healthy individuals; the follicles were separated, plated on coverslips coated with collagen G, and cultivated in McCoy 5A Medium in a CO2-incubator at 37 degrees C. Light and electron microscopy after 1, 2, 3, and 6 weeks showed selective and progressive cell growth with keratinocyte differentiation, producing multilayered cultures of cells joined with fully developed desmosomes. Three distinct patterns of differentiation, leading to the formation of an incomplete horny layer, were seen. The particular arrangement of tonofilaments, the considerable amounts of cytoplasmic glycogen, and the absence of malpighian differentiation were ultrastructural indicators of the follicular origin of the cultured cell population, which most likely grew from the outer root sheath of the hair. This technique may provide a promising model on which to base further studies of hair biologic processes and hair growth.