Growth hormone is the classical anabolic hormone which promotes organ growth after binding to somatogenic target cell receptors, present in various target tissues. The present study elucidated the pharmacological characteristics in vitro and in vivo of human growth hormone and a recently identified by-product of a recombinant human growth hormone preparation; i.e. a trisulfide-containing (cys 182-cys 189) hydrophobic, folding derivative of growth hormone, hydrophobic derivative-growth hormone. Standard growth hormone and hydrophobic derivative-growth homone possessed similar characteristics in vitro, both as regards binding to the somatogenic receptor on the human IM-9 cell line, and the prolactin receptor-mediated proliferation of rat Nb2 cells. This indicates that no change occurs in the binding characteristics in spite of a change in conformation of the molecule. Using an ELISA assay that detected standard and hydrophobic derivative-growth hormone equally well, the plasma pharmacokinetical profiles of the preparations following a single intravenous or subcutaneous dose were indistinguishable. Thus, following initial disposition of hydrophobic derivativegrowth hormone and standard growth hormone into a volume, Vl, of one to two times the plasma volume, almost 90% of either compound disappeared from plasma during the α-phase of the plasma decay curve. Similar half-lives of 4-5 min. were found for hydrophobic derivative-growth homone and standard growth hormone during this phase, indicating rapid removal of drug from the circulation. Also, the AUC and Cmax values for standard and hydrophobic derivativegrowth hormone did not differ following intravenous or subcutaneous administration. Examination of whole body autoradiograms following dosing with 125I-labeled standard growth homone revealed intensive staining of the liver and renal cortex, suggesting elimination via these organs. Finally, and concording with the in vitro and in vivo bioequivalence of hydrophobic derivative-growth hormone and standard growth hormone, anabolic action of the hydrophobic derivativegrowth hormone in the tibia test was shown to be fully retained. In conclusion, pharmacological profiling has been performed for standard growth hormone and a cys 182-cys 189 trisulfide-containing hydrophobic growth hormone derivative, and no differences were detected between the two in vitro or in vivo.