GHBP is the ectodomain of GHR, cleaved from membrane-bound GHR in man and rabbits, while in rodents it is derived from an alternatively spliced mRNA. GHBP circulates in nanomolar concentrations, sufficient to complex approximately 50% of plasma GH (Baumann et al. 1988). GHBP can compete for GHR binding, inhibiting GHR signaling (Lim et al. 1990), and generates 'unproductive' heterodimers with GHR at the cell surface (Ross et al. 1997), but GHBP can also increase GH biological activity by prolonging its half-life (Baumann et al. 1987). The net effect of GHBP may depend on the relative concentrations of circulating GH and GHBP (Lim et al. 1990, Barnard & Waters 1997), the overall effect is postulated to be stabilization of GH signaling (Veldhuis et al. 1993). GHBP appears to be positively linked to GH action. It has been suggested that plasma GHBP levels reflect tissue concentrations of GHR, but this remains to be proven.