The monoclonal antibody 2F1 reacts with human soluble leptin receptor (sLR) in plasma. Leptin is a cytokine that is primarily expressed by adipose tissue. Leptin controls food intake by its interaction with the leptin receptor in the brain. Leptin action is mediated and controlled by the leptin receptor, a class I type cytokine receptor. sLR is generated by proteolytic cleavage of membrane-anchored receptors. This indicates that the leptin receptor might have other functions besides transduction. Binding of leptin with sLR increases the bioavailability of leptin in plasma, but also decreases the binding of leptin to membrane bound leptin receptors. For example, when comparing obese and lean individuals, plasma levels of sLR are significantly decreased whereas leptin levels are significantly increased. The monoclonal antibody 2F1 specifically reacts with sLR with a molecular mass of 180kD. The 2F1 antibody can be used to measure in both free sLR and sLR bound to leptin in plasma.