Class-specific RFs were measured in serum samples obtained from 810 individuals in seven age categories between 20-100 yr old, who were randomly selected from the general population. There was a decline with age of the mean IgG-RF levels in the higher age categories, no change of the mean IgA-RF levels and a clear increase of the mean IgM-RF levels in the higher age categories. In most age categories, the percentage of increased levels of all three RF isotypes was higher in the general population than in the healthy control population, consisting of 100 blood donors. These observations suggest that reference values for serum RF should be age-specific and based on sera of individuals randomly selected from the population.