Plectin is a high-molecular-weight cytoskeleton-associated protein that was initially identified in intermediate filament (IF) enriched fractions of rat C6 glioma cells, At the cellular level, plectin has been found to associate with IF networks and IF-associated structures that are involved in cell-cell and cell-substrate adhesions, IFAP300 is an IF-associated protein that was initially identified in hamster cells by a monoclonal antibody directed against a high molecular weight protein present in IF-enriched cytoskeletal preparations. Plectin and IFAP300 display similar distribution patterns within cells as determined by immunofluorescence. Based upon this and the finding that their biochemical properties are similar, it has been suggested that they may actually be orthologous proteins. In this paper we demonstrate that this is the case. Cloning and sequencing of most of the hamster plectin cDNA demonstrates that plectin is found in hamster cells and that its sequence is highly conserved between species. Using immunological crossreactivity, epitope mapping, and immunoelectron microscopy, we show that IFAP300 is actually the hamster ortholog of plectin.