1. The thin cuticle of Acarus siro contains a relatively large quantity of sulphur.
2. The sulphur does not form disulphide linkages between cysteine molecules as it does in keratin.
3. The presence of lysine amongst the products of hydrolysis shows that this cuticle is not tanned by linkage of quinone and amino groups.
4. The experimental data support the view that the cuticle is tanned by reaction of quinones and sulphydryl groups.
5. It is suggested that the quinone involved is a paraquinone, probably derived from phenylalanine.