Sera from 107 blood donors ranging in age from 18 to 66 years were tested by indirect immunofluorescence for titers of various autoantibodies. These were matched for sex in diffferent age brackets. Antinuclear antibody (ANA) titiers of 10 or more appeared in 45 per cent of the sera examined. However, only 5.6 per cent had ANA titers of 80 or more, and only one subject had a titer of 160. ANA titers of 80 or greater were only found in the oldest age group (51 years or older). In those subjects with detectable ANA, 57.7 per cent were female and 32.8 per cent were male. The prevalence of thyroid cytoplasm and gastric parietal cell antibodies also appeared both age and sex related, while smooth muscle antibodies did not. Anti-DNA antibody, pemphigus antibody, bullous pemphigoid antibody, and mitochondrial antibody were detected in none of the sera tested. No relationships were seen in pairs of autoantibodies.