Soluble forms of growth hormone receptors, growth hormone-binding proteins (GH-BPs), with molecular weights of 60 and 55kDa were found to be constitutively released from human IM-9 cells. The release of the GH-BPs was not inhibited by inhibitors of protein synthesis (cycloheximide) and transport (brefeldin A). Down-regulation by human growth hormone or trypsin pretreatment of surface growth hormone receptors abolished the GH-BP release, suggesting that cell-surface human growth hormone receptors are involved in the GH-BP release. Several inhibitors of serine-, thiol-, and acid-proteases did not affect the GH-BP release. EDTA efficiently blocked the GH-BP release. This inhibition by EDTA was restored by addition of Mg(2+) and Co(2+). These results suggest that human GH-BPs are constitutively released by proteolytic cleavage of cell-surface growth hormone receptors by a metalloprotease.