OBJECTIVE Fatty acid-binding proteins (FABP) make up a family of cytosolic proteins of unknown function found in many tissues, including the embryonic chick retina. To further understand its function in the developing retina, the protein from embryonic chicks was purified and characterized. The tissue distribution of this protein in embryonic and hatched chicks was determined using polyclonal antiserum prepared against this FABP. METHODS FABP was purified by column chromatography and native gel electrophoresis; denaturing gel electrophoresis and isoelectric focusing were used to characterize the protein. Polyclonal antiserum prepared in rabbits demonstrated immunoreactivity with a single protein from retina cytosolic fraction. Tissue distribution of retinal FABP was determined by immunoblotting and localization of FABP in eyes of E15 and adult chickens was studied by immunohistochemistry on paraffin sections. RESULTS Retinal FABP has a molecular weight of approximately 15,100 and an isoelectric point of 4.4. An immunoreactive protein of the same apparent molecular weight is present in the brain, heart, adipose tissue, and red muscle; a trace of FABP is present in white muscle, lung, and liver. There is no apparent cross-reactivity with a protein in intestine or in retinal pigment epithelium/choroid. Immunostaining of sections of the adult eye at the region of the ora serrata revealed little staining of the retina, but intense staining of the nonpigmented layer of the ciliary epithelium. CONCLUSIONS Retinal FABP resembles the heart subtype of FABP (H-FABP) with respect to molecular weight, isoelectric point, and tissue distribution. However, as retinas do not use fatty acids for beta-oxidation, a process H-FABP is believed to participate in, there must be some other function for FABP in the chick eye, especially at early stages of development. In the mature retina, this function is apparently not needed, because there is little staining of the retina in adults but obvious immunoreactivity in the ciliary epithelium, implying some other role for FABP in this tissue.