An extension of the immunological theory of ageing is proposed according to which mutator genes specifically controlling histocompatibility genes become derepressed at the end of the reproductive period. A progressive, exponential loss in self-recognition homœostatic mechanisms and in immunological competence follows this derepression. The senile phase of ageing and the " diseases of ageing" ensue. Such a mechanism for ageing is consistent with ideas about the developmental biology of the immune system, with the increasing knowledge about the complexities of the HL-A and H-2 histocompatibility systems, and with numerous other experimental and observational data.