A quantitative histochemical study was carried out on the distribution of protein thiol and disulphide groups in the telogen follicle of mice. The histochemical demonstration of reactive groups was achieved by a method first described by Sippel (1973). The histological observations support former views on trichilemmal keratinisation that trichilemmal cells are the main site of thiol groups and that the hair club itself appears to be the most prominent site of disulphide cross linkages. The histophotometric data reveal an increase in thiol-disulphide conversion which coincides with the development of a succeeding hair generation in the trichilemmal sac of the telogen follicle. On the basis of this observation, it is suggested that trichilemmal keratinisation is a causal factor in loosening the old club hair from its trichilemmal sac, a process which is probably similar to the sloughing of cornified epidermal cells.