Serum from 14-day-old chickens rendered tolerant at hatching with 1250 mg bovine serum albumin (BSA)/KBW was found to suppress an adoptive secondary response to BSA and an in vitro response to sheep red blood cells (SRBC). Chromatographic separation resulted in three fractions: I, II, and III. The suppressive activity was found to be contained in fraction I (m.w. >1 × 106) and fraction III (m.w. <2.5 × 104). Analysis of fraction I by affinity chromatographic techniques revealed that it was composed of soluble BSA-antibody complexes and their removal removed the suppressive activity. Analysis of peak III revealed that the suppressive activity could only be removed by BSA and anti-Line 3 antiserum columns.