We have determined the complete primary structure for the carboxyl-terminal peptides of mouse alpha 1(IV) and alpha 2(IV) collagen; which have 229 and 227 amino acids, respectively. The amino acid sequences are 63% identical and conservatively substituted in 28 positions. A striking feature of these peptides is that the first half of each sequence is homologous with the second half, 37% in alpha 1(IV) and 36% in alpha 2(IV). These results suggest that the carboxyl-terminal peptides of type IV collagen are closely related in their structure and evolution. Presumably, they were first derived by internal duplication of a common ancestral DNA sequence which later, by gene duplication, gave rise to the two different but homologous carboxyl-terminal peptides of type IV collagen.