The described Enzyme-Linked-Immuno-Sorbent-Assay (ELISA) is intended for the quantitative determination of total sRANKL (human) in serum and plasma. It is for in vitro diagnostic use only. The assay detects free as well as OPG-bound sRANKL in serum and cell culture supernatants. Free sRANKL can be mathematically estimated when the assay is performed once with an excess of OPG (free and OPG-bound sRANKL are determined), and then without any addition of OPG (only OPG-sRANKL-complexes already present in the sample are determined).