1. The two major polypeptides (P1 and P2) of erythrocyte-membrane spectrin were isolated by preparative polyacrylamide-gel electrophoresis. 2. The two polypeptides were shown to possess similar amino acid compositions, both with the characteristically high glutamate and leucine contents of the parent spectrin. 3. The tryptic-peptide 'maps' of the two polypeptides were prepared by a combination of t.l.c. and electrophoresis. 4. Radioactive peptides were prepared by [14C]carboxymethylation and chloramine-T-catalysed [125I]iodination. 5. 'Maps' of both sets of peptides demonstrate a marked similarity between the two polypeptides. 6. These new data confirm earlier evidence for the similarity of the two chains. 7. The number of peptides in the 'maps' of carboxymethylated peptides suggest that polypeptides P1 and P2 are not aggregates.