IgG anti-gamma-globulin factor in rheumatoid arthritis sera detected by mixed agglutination test

In mixed agglutination tests with patient's sera and sensitized sheep erythrocytes on a glass slide, IgG rheumatoid factor was dected in 15 of 30 rheumatoid arthritis, 3 of 4 progressive systemic sclerosis and 1 of 3 systemic lupus erythematosus sera as well as in 3 of 4 synovial fluids of rheumatoid patients but in none of 10 normal sera. The IgG nature of this factor was confirmed by demonstrating this factor in IgG fractions of positive sera eluted from DEAE-cellulose and its insensitivity to mercaptoethanol.