Ostrich (Struthiocamelus), the largest flightless bird's and the flying bird (Gallus gallus) hen's egg Riboflavin Binding Protein (RBP) was isolated and purified from their eggs. First, the purification of RBP was undertaken in two steps, using DEAE-Sepharose ion exchange chromatography. Second, the purified (RBP) sample was obtained using gel filtration on Sephadex G-100. The holoprotein had an absorption spectrum characteristic of Flavoproteins. The purity of the RBP, was confirmed by SDS slab-gel electrophoresis, where the isolated RBP migrated as a single band. The mobility of the RBP with that of standard mol.wt proteins marker suggested that the mol.wt was 54 kda (Ostrich) and 29 kda (Hen). Against these RBP's antiserum was raised in Rabbits and this antiserum showed immunological cross reactivity -precipitin lines between the two compounds i.e flightless bird and flying bird RBPs. Hence the study proves that, the largest flightless bird (Ostrich) and flying bird (Hen) are immunologically relatives and phylogenetically different from each other.