Leptin is the 16 kDa product of the obese gene secreted by white adipose tissue in proportion to energy intake. In several species, plasma leptin levels correlate with, and may be and endogenous signal of, the extent of body fat reserves. Leptin acts via the hypothalamus to regulate feed intake in rodents and may control gonadal activity through this same axis. Significantly, the mutant ob/ob mouse lacks functional leptin and is both hyperobese and infertile. Leptin may be a useful diagnostic and/or therapeutic aid in high-yielding dairy cows in which infertility is rising inexorably and is anecdotally linked to the scale of the early post-partum negative energy balance. Our objectives were to recombinantly synthesise, and raise antibody to, a leptin-protein conjugate and to develop a useful and specific RIA for ruminant leptin.