The reaction of rheumatoid factor with bovine IgG fragments obtained by papain digestion and by reduction.

1. The fragments produced from bovine IgG by papain digestion and by reduction were similar to those from human IgG. 2. The Fab and Fc fragments produced by papain digestion and the light chains produced by reduction of bovine IgG did not react with rheumatoid factor. 3. The heavy chains produced by reduction reacted with rheumatoid factor, but did not inhibit the reaction of whole bovine IgG with rheumatoid factor. 4. The difference between the reactions of human and bovine Fc fragments with rheumatoid factor is discussed. It is suggested that the major site in bovine IgG with which rheumatoid factor combines is on the heavy chain in the region of the junction of the Fab and Fc subunits.