As part of a long term prospective study, sera from 3492 subjects, over 90% of the adult population of the rural Australian town of Busselton, were tested by indirect immunofluorescence for various autoantibodies, and by latex particle agglutination for rheumatoid factor. Autoantibodies were detected in 21·6% of all subjects and the overall prevalence increased with age in both sexes. A dip in the age prevalence curve of autoantibodies amongst subjects aged 75–80 years preceded a terminal sharp rise. The greater prevalence in females, 27·5%, than in males, 13·7%, was accounted for by autoantibodies to nuclei, gastric parietal cells and thyroid epithelial cells; the prevalence of smooth muscle antibody (SMA) and rheumatoid factor (RF) was similar in females and males, and the prevalence of SMA did not increase with age. Thus development of `age-sex dependent' autoantibodies, to gastric and thyroid antigens, may have a considerable genetic component, whereas the non `age-sex dependent' autoantibodies, SMA and RF, may have a considerable extrinsic component, e.g. infection. Autoantibodies were associated in two groups, a `nuclear' group and a `thyrogastric' group; these groups were neither positively nor negatively associated, implying independent genetic or other causes for their appearance.