Rheumatoid factors (RF) detected by agglutination methods (latex fixation test, haemagglutination test) were found in a group of patients with rheumatoid arthritis which started in advanced age (RAVV) less frequently than in a group with RA in middle age (RAMV). The author describes the ELISA method for assessment of RF which makes it possible to estimate immunoglobulin classes RF (IgM-RF, IgA-RF, IgG-RF). By this method RF were detected in class IgM in 15% of patients and in some immunoglobulin classes in 20 per cent where are RF negative when assessed by agglutination methods. The ELISA method detects probably also so-called "hidden" RF. In the group of healthy subjects above 60 years at least one class of RF rises to 17.5%. On correlation with the clinical status an association was found with the degree of affection according to the functional score, with the presence of rheumatoid nodes and muscular affection in RA. Patients with RA treated with Methotrexate and Sulfasalazine tend to have declining IgG RF levels.