Studies of leptin in large domestic ruminants have been limited to measurements of gene expression because methods to measure circulating levels are not available. To develop a bovine leptin radioimmunoassay, we produced recombinant bovine leptin and used it to immunize rabbits, and to prepare bovine leptin tracer and standards. A single antiserum with sufficient affinity and titer was identified. Using this antiserum, logit-transformed binding of 125 I-labeled bovine leptin was linearly related (R 2 = 0·99) to the log of added bovine or ovine leptin between 0·1 to 2·0 ng. Serial dilution of bovine and ovine plasma, chicken serum and bovine milk gave displacement curves that were parallel to those of bovine or ovine leptin. Recoveries of external addition of bovine leptin in ewe and cow plasma ranged between 94 and 104%. Plasma leptin concentration measured by this assay was directly related to the plane of nutrition in growing calves and lambs. At 11-14 weeks of age, ewe lambs had a higher circulating leptin concentration than ram lambs. Finally, plasma leptin concentration was linearly related to the fat content of the empty carcass in growing cattle and to body condition score in lactating dairy cows. We conclude that circulating leptin in sheep and cattle is increased by fatness and plane of nutrition, consistent with results in humans and rodents. This assay provides an important tool to investigate mechanisms that regulate plasma leptin in cattle and sheep.