Thermogenin is the purine-nucleotide binding polypeptide in brown adipose tissue 
mitochondria (Mr 32 000) which confers upon these mitochondria the ability to 
produce heat. An enzyme-linked immunosorbent assay (ELISA) has been developed to 
demonstrate and quantitate the occurrence of thermogenin antigen in small 
amounts of tissue, and thus to characterize different depots of fat tissue as 
white or brown. The extreme sensitivity of the method allows determination of 
thermogenin in samples equivalent to less than 1 mg tissue. The results indicate 
that thermogenin seems to be exclusively localised in brown fat mitochondria (as 
compared to white fat, liver or heart muscle mitochondria), and thermogenin 
antigen could only be found in brown adipocytes (as compared to white 
adipocytes). Thus, brown and white adipose tissue are probably ontogenetically 
different.