A specific protein exhibiting immunological cross-reactivity with chicken egg-white riboflavin carrier protein was detected by radioimmunoassay in the pregnancy sera of bonnet monkeys (Macaca radiata). This protein, which is capable of binding [14C]riboflavin, was purified by gel filtration on Sephacryl S-300, fast protein liquid chromatography on a Mono Q anion exchanger and chromatofocusing on Mono P columns. The isolated primate carrier protein was similar to its avian counterpart in terms of physicochemical characteristics, such as isoelectric point (pI less than or equal to 4), electrophoretic mobility, molecular weight (approx. 36,000) and ligand binding. These findings may account for the extensive immunological cross-reactivity observed between the two proteins and suggest that the two vitamin carriers may have similar function in terms of embryonic vitamin nutrition.