Leptin circulates in murine serum in a free and a bound form. As shown in humans, a soluble leptin receptor (sOB-R), which modulates the effects of its ligand, circulates in murine blood. The aim of our study was to determine abundance and biochemical nature of this protein. For the quantification of sOB-R we developed a ligand-immunofunctional assay (LIFA) which is based on both, leptin binding and immunological recognition. The use of this LIFA revealed that during late gestation sera of pregnant mice had a approximately 290-fold higher level of sOB-R than non-pregnant animals. As investigated by size exclusion chromatography these mice sera demonstrated a co-elution of their leptin binding activity with leptin immunoreactivity and levels of sOB-R measured by LIFA. Therefore, it has to be concluded that sOB-R represents the major leptin binding activity in murine circulation. The molecular analysis of sOB-R by Western blot and by cross-linking with 125I-leptin in sera of pregnant and non-pregnant mice demonstrated two different isoforms of sOB-R, which were capable of leptin binding. The sOB-R in serum migrated at a molecular weight of 150kDa in pregnant and only of 120kDa in non-pregnant animals. Deglycosylation of these isoforms led to sOB-R molecules which were found at the same molecular weight in SDS-PAGE. This finding indicates that both isoforms differ only in the degree of their glycosylation. In conclusion, the non-pregnant and the pregnant states are accompanied by differently glycosylated isoforms of sOB-R whose physiological relevance remains to be determined.