1. A solid-phase radioimmunoassay is described for the estimation of the uncoupling protein content of human brown adipose tissue mitochondria, as an index of thermogenic capacity.

2. The concentration of inner mitochondrial membrane uncoupling protein was measured in brown adipose tissue samples from 48 individuals who died suddenly.

3. The uncoupling protein content of axillary adipose tissue was greater than that of perirenal adipose tissue.

4. Variations in brown adipose tissue uncoupling protein content, which would be consistent with changing thermogenic requirements and capacity, were observed in different groups of subjects. Significantly lower concentrations were found in adults and in pre-term and stillborn infants than in older infants and children.

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