Egg yolk has been reported to inhibit B12 absorption less than egg white suggesting that different vitamin B12 binding proteins may be present in egg white and egg yolk. Using gel-exclusion chromatography we found that the mean MR for the B12 binding protein derived from egg yolk was 125,000, whereas that derived from egg white was 97,750. Heat treatment of the apoprotein differentially reduced the binding capacity of egg yolk and egg white in a time-dependent manner with the greatest decrease in binding capacity occurring with egg white. In contrast, heat treatment of the holoenzyme delineated the egg yolk as the more labile. These studies suggest that egg yolk and egg white contain distinct R binders which could explain the differential B12 absorption from egg yolk and egg white.