1. The capacity of rheumatoid-arthritis serum (RAS) to influence the agglutinability of sensitized sheep-erythrocytes was found to be unrelated to its content of sheep-cell agglutinins. Removal of such agglutinins by absorption with sheep cells did not alter the titer of the serum for sensitized sheep-cells. Absorption with sensitized cells likewise did not readily reduce the titer of the serum for sensitized cells although agglutinated cells remained so even after repeated washings.

  2. Extraction of RAS with lipoid solvents failed to destroy the capacity of the serum to agglutinate sensitized sheep cells.

  3. RAS which markedly increased the specific agglutination of sheep cells did not show a similar effect on the specific hemolysis of the cells.

  4. RAS not only agglutinated sheep cells sensitized with rabbit anti-sheep-cell serum but also agglutinated sheep cells sensitized with rabbit anti-goat-cell serum and with rabbit anti-guinea-pig-kidney serum. Sheep cells sensitized with serum from patients with infectious mononucleosis and from normal persons were agglutinated to about the same degree as unsensitized cells.

  5. The agglutination of beef cells and goat cells by homologous rabbit immune sera was markedly increased by RAS, whereas the agglutination of human group 0 cells and rat cells was not significantly affected.

  6. Sheep cells sensitized with rabbit anti-sheep-cell serum which had been absorbed with guinea-pig kidney, or with raw or boiled beef cells, were agglutinated in RAS to the same degree as cells sensitized with unabsorbed serum. Sheep cells sensitized with homologous immune serum absorbed with goat cells were agglutinated less strongly in RAS than cells sensitized with unabsorbed serum.

  7. Absorption of agglutinins for autoclaved group A streptococci from RAS did not affect the titer of the serum for sensitized sheep cells.

  8. The possible relation of the phenomenon to the agglutination-intensifying effect of serum-globulins generally is discussed.

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