Two soluble non-specific suppressive factors were isolated by gel chromatography from the serum of chickens tolerant to bovine serum albumin (BSA). Fraction I was found by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) to contain five proteins with mol. wts from 200,000 to 900,000 unreduced and six proteins that ranged from 70,000 to 180,000 when reduced. The composition was primarily protein, with a small amount of carbohydrate and lipid. One component was identified as IgM, one as a lipoprotein. The others remain unidentified. No evidence of the tolerogen, BSA, was found. The second factor, fraction III of both tolerant and normal serum, was found by SDS-PAGE to have two proteins with mol. wts of 54,000 and 78,000 in the unreduced form and contained a very small amount of protein, a large amount of carbohydrate, and no detectable lipid. Normal fraction III had two proteins with mol wts of 54,000 and 65,000 when reduced, whereas reduction did not affect tolerant fraction III. A soluble suppressive factor, secreted by cultured adherent cells of tolerant chickens, was found to be comparable to fraction III in molecular weight. Comparison of these results with those of other reported suppressive factors is made and possible identities of the suppressive components of fractions I and II are discussed.