From adult temporal bones with clinically normal ears mucous lining specimens were taken and investigated microscopically. Comparing these specimens with biopsies from the tympanon of patients following tympanoplasty, we could detect an over-all correspondence of epithelial and subepithelial cell structures of normal tympanic mucosa and of ears operated upon because of chronic inflammation. These findings suggest that the middle ear epithelium is an only partial developed mucociliar - and not an pluripotential - epithelium, arising by ingrowth of respiratory mucosa of the nasopharynx. It reacts upon every exogenic or endogenic irritation always in direction to complete differentiation. This differentiation is caused by stimulation of the O2-metabolism following such various proceedings as traumatisation, inflammation, otospongiosis activity, experimental tympanoplasty or explanation into tissue culture. The generally reversible process should not be defined as metaplasia.