We have used scanning transmission electron microscopy to elucidate the question of how intermediate filament (IF) subunits of widely differing mass can all form morphologically similar IF. From scanning transmission electron micrographs, the distributions of mass were determined for three types of epidermal keratin IF reassembled in vitro from mixtures of subunits with substantially different masses, viz., "light" and "heavy" human keratins with [Mr] = 50,000 and 56,000, respectively, and mouse keratins of [Mr] = 63,000. Their principal assembly products were found to average 22, 25, and 29 kdalton/nm, respectively. These densities, which correspond to immature "minimal form" IF (Steven, A. C., J. Wall, J. Hainfeld, and P. M. Steinert, 1982, Proc. Natl. Acad. Sci. USA., 79:3101-3105), are directly proportional to the average subunit masses. The human keratin IF (but not those of mouse) also contained minor amounts (15-20%) of more massive polymers averaging 33 and 35 kdalton/nm, respectively, which probably represent mature IF. Taken together with earlier results on IF of other subclasses, these results indicate that the average linear density of IF scales according to the average mass of their constituent subunits, both for "minimal form" and for mature IF. As underlying mechanism for this homology, we propose that the fundamental building-blocks of all these IF contain a common structural element whose packing within the various IF is likewise conserved and which specifies the overall structure. The variable amounts of mass in the nonconserved moieties account for the observed proportionality. This scheme fits with amino acid sequence data for several IF subunits that have revealed, as a likely candidate for the common element, an essentially conserved alpha-helical domain, contrasting with the highly variable sequences of their non-alpha-helical terminal domains.