Using a synthetic 26 base pair (bp) oligonucleotide, we have screened a rabbit liver cDNA library in lambda gt11 and isolated a 1000 bp DNA clone corresponding to cytochrome b5 mRNA. The clone contains the complete coding region and long 5' and 3' non-translated regions. The derived amino acid sequence (sequence; see text) agrees with published sequences, and confirms that the amino acids 62 and 104 are asparagine and aspartic acid, respectively.