Total RNA was extracted from a variety of rabbit tissues and reverse transcribed for use in the polymerase chain reaction technique. Using primers designed to amplify the membrane-bound liver cytochrome b5 cDNA, products of two sizes were observed. Both hybridized strongly to a radiolabelled liver cytochrome b5 probe. Sequencing confirmed that the two types of cDNA product encoded the membrane-bound and the soluble forms of b5. Messenger RNA corresponding to the soluble cytochrome was detected in the lung, gallbladder and the adrenal gland, as well as in reticulocytes and bone marrow. This was an unexpected finding since the protein has been isolated only from erythrocytes. In contrast, membrane-bound cytochrome b5 mRNA was detected in all tissues tested, suggesting that the corresponding protein is ubiquitous in tissue distribution.