Therapeutic pegylated growth hormone antagonists

A method for treating diseases or conditions responsive to human growth hormone receptor antagonists, comprising administering to the patient an effective amount of a human growth hormone receptor antagonist, comprising:

• (a) human growth hormone receptor antagonist G120K having a DNA sequence of SEQ ID NO:
• 3 and an amino acid sequence of SEQ ID NO:
• 4, wherein two amino acids of human growth hormone receptor antagonist G120K have been mutated to cysteine, wherein the two amino acids mutated to cysteine are T142 and H151;
• and
• (b) a polyethylene glycol molecule conjugated to each substituted cysteine in the human growth hormone receptor antagonist G120K mutant, wherein the polyethylene glycol molecules conjugated to the two amino acids mutated to cysteine are two 4.5 kDa branched polyethylene glycols each containing three carboxylate anions, wherein the polyethylene glycol molecule is prepared by step-wise organic chemistry and is a substantially pure single compound, and wherein the polyethylene glycol molecule is a branched structure.