The effect of acclimation temperature on the concentration of the mitochondrial 'uncoupling' protein (M r 32 000) from brown adipose tissue of mice has been investigated. The uncoupling protein was measured by a specific radioimmunoassay. Between 33 o C (thermoneutrality) and -2 o C there was a progressive increase with decreasing environmental temperature in the amount of uncoupling protein. For mice at -2 o C the mitochondrial concentration of the protein was 9-times higher than at 33 o C, while the total amount of the protein in interscapulaar brown adipose tissue was estimated to be nearly 80-times greater at -2 o C compared to 33 o C.