Filensin is a lens-specific intermediate filament protein, expressed in the lens fiber cells but not the lens epithelium. Using antibodies to filensin and the other lens intermediate filament proteins, vimentin and CP49, the codistribution of filensin with CP49 and independence of this network from the vimentin network was confirmed. Monoclonal and polyclonal antibodies to peptides and specific subdomains of filensin were used to follow changes in the subcellular distribution of filensin during bovine lens fiber cell differentiation. Filensin is shown to be extensively processed during lens fiber cell differentiation to give protein fragments derived from distinct protein domains, one corresponding to the N-terminal non-alpha-helical/and rod domain and the other to the C-terminal non-alpha-helical tail domain. Immunoblotting analysis using anti-filensin peptide polyclonal antibodies suggested that the two fragment sets arose separately. Residues 331 to 430 in filensin have been identified as an important region in the processing pathway(s). Our results clarify previous confusion in the literature regarding the processing of filensin which arose because of the similar relative electrophoretic mobilities by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) of the different fragment sets. The predicted secondary structure characteristics of the different domains of filensin suggests different functions for the two fragment sets to give filensin a dual role in the lens. This suggestion is supported by the subtly different subcellular distributions in the peripheral and mature fiber cells of the two filensin fragment sets.