Ligatin, a filamentous protein previously found in suckling rat ileum, has been purified from plasma membranes of embryonic chick neural retina. The isolated plasma membranes are covered in part by 4.5-nm filaments that can be released from the membranes by treatment with Ca++. Subsequent dialysis against EGTA followed by sieve chromatography results in purification of the 10,000-dalton ligatin monomer. When labeled either with radioisotopes or with fluorescamine, the monomer is shown to electrophorese as a single discrete band in polyacrylamide gels. However, during standard fixing and staining procedures it diffuses from the gels and thus is not visualized. Ligatin's amino acid composition is distinguished by its high content of polar residues, especially Glx and Asx, and by the presence of phosphorylated serine. Upon re-addition of Ca++, purified ligatin monomers polymerize to form filaments 3 nm in Diam, identical to those formed by purified ileal ligatin. However, in both retina and ileum, the filaments observed on plasma membranes are greater than 3 nm in Diam. In ileum, this enlargement results from ligatin's function as a baseplate for the attachment of another protein, a beta-N-acetylhexosaminidase, to the cell surface. In retina, a corresponding difference in diameter between filaments seen in vivo and those formed from repolymerized ligatin alone and the co-solubilization of other proteins with ligatin suggest that ligatin may also function there as a baseplate for other cell surface proteins. The proteins associated with ligatin in retina differ morphologically from beta-N-acetylhexosaminidase and do not possess this enzymatic activity.