<p>The adsorption behavior of recombinant human growth hormone (r-hGH) on cationic (PS⁺, amidine) and anionic (PS⁻, sulfate) polystyrene surfaces was investigated under varying solution pH conditions (surface charge), ionic strengths (0.0075&#xa0;M and 0.075&#xa0;M), and solvent dielectric constants to elucidate the adsorption mechanism governed by physicochemical interactions. In addition, desorption upon dilution was also examined. The studies utilized ¹²⁵I-labeled r-hGH and demonstrated that adsorption of r-hGH onto both PS⁺ and PS⁻ surfaces was influenced by solution conditions, ionic strength, and solvent dielectric constant. Marked electrostatic repulsion accompanied by decreased adsorption was observed on PS⁺ surfaces at pH 2.5 and on PS⁻ surfaces at pH 7.2. Adsorption was highest near the isoelectric point of r-hGH but decreased with increasing ionic strength. PS⁺ and PS⁻ surfaces showed significantly different adsorption profiles, resulting from the combined effects of hydrophobic and electrostatic interactions involving r-hGH, the change in r-hGH confirmation in solution, and the structural characteristics of the surface adsorbed protein. Studies were also carried out to assess the effect of the presence of proteins and surfactants at an ionic strength of 0.0075&#xa0;M over a pH range of 2.5–7.2. Under conditions where both proteins inhibited adsorption, β-casein exhibited a greater inhibitory effect than BSA. Surfactants exhibited concentration dependent effects, with Tween 20 producing stronger inhibition than Pluronic F-68. Overall, these results demonstrate that r-hGH adsorption onto charged polystyrene surfaces is influenced by a complex interplay of charged interactions, hydrophobic effects, competitive adsorption, surfactant effects, and protein conformational stability.</p> Graphical Abstract <p></p>