We have isolated clones of a processed pseudogene of mouse t complex polypeptide 1 (Tcp-1) and determined the nucleotide sequence of the pseudogene. The pseudogene was 1363 bp long and had no intron. The Tcp-1 pseudogene had 88.4% or 88.3% nucleotide identity to the mouse Tcp-1 cDNA of wild-type (Tcp-1b) or t haplotype (Tcp-1a), and 87.5% identity to the rat Tcp-1 cDNA. On 12 nucleotide positions where the open reading frames (ORFs) of mouse Tcp-1b and Tcp-1a cDNAs have bp substitutions, the Tcp-1 pseudogene had 6 bp identical to Tcp-1b, 5 bp identical to Tcp-1a and 1 bp not identical to neither. On ten amino acid positions where TCP-1B and TCP-1A polypeptides have substitutions, deduced amino acids of the Tcp-1 pseudogene had four amino acids identical to TCP-1B, five amino acids identical to TCP-1A and one amino acid identical to neither. These results suggest that the ancestral mouse Tcp-1 gene would have had no significant difference between the resemblance to Tcp-1b and that to Tcp-1a before they were diverged and that amino acids of TCP-1B and TCP-1A would have been substituted in similar high rates.