Cell aggregates formed during the first 2 days of culture of cells derived from hair follicle tissue of young rats. Aggregates occurred at an accelerated rate at higher cell densities, and contained a high proportion of epithelial cells although a variable proportion of mesenchymal (fibroblast) cells were present. Citrulline was detected in the cultured cell proteins. Electrophoretic analysis of the proteins showed the presence of hair cortical keratin in the cultures, but these proteins were not synthesized during the culture period, in conflict with previous findings. Protein synthesized by the cultured cells was not of inner root sheath or epidermal origin and the only protein which could be identified was actin. It is concluded that the present culture system is unsuitable for the study of keratinization as occurs in the intact hair follicle.