Familial occurrence of three types of autoantibodies, viz. rheumatoid factors, antinuclear antibodies and thyroid autoantibodies, was investigated in two population series: twins from the Finnish Twin Registry representing the adult twin population living in an urban area (76 monozygotic and 82 dizygotic same-sex twin pairs) and siblings from a middle-aged rural population subject to an ongoing epidemiological study (311 sibling groups comprising 801 subjects and 584 married couples). The observations were compatible with the concept of two components in the above autoantibody system. The major component is determined predominantly by environmental factors and is manifested with increasing age. The autoantibodies belonging to the minor, familial component occur in somewhat higher titers, exhibit certain specificity features of their own (RF reacting with rabbit gamma globulin), and are associated with "autoimmune" diseases.