Very hydrophobic proteins are especially demanding regarding the development of liquid pharmaceutical formulations, mainly due to their low solubility which makes them prone to aggregation. Aggregates may elicit immune response in patients and their acceptable content in the final drug product is strictly limited. The most common method for protein stabilization in liquid formulations is addition of excipients (e.g. sugars). Another successful approach could be PEGylation. Protein PEGylation is primarily being introduced in cases where improved pharmacokinetic and pharmacodynamic properties arising from the prolonged elimination half-life are envisaged. But protein conjugation to the polyethylene glycol (PEG) chain also brings other benefits, such as increased solubility, which is particularly important when dealing with very hydrophobic proteins. In the MSc thesis we have performed a stress stability study of a highly hydrophobic model protein and tested a protective role of PEGylation on aggregation. Additionally, the effect of sugar excipient (trehalose) was tested. Our initial hypothesis was that PEGylation will improve stability of the model protein. The model protein (Protein_t) in an optimised liquid pharmaceutical formulation containing trehalose was compared to the PEGylated protein (PEGprotein) in two different liquid pharmaceutical formulations, with (PEGprotein_t) and without (PEGprotein_a) trehalose. The stability studies included temperature stability (-60 °C, 4 °C, 25 °C and 40 °C), freeze/thaw cycles and sheer forces (shaking). Stability of the tested samples was evaluated with standard HPLC techniques RPC and CEC. Aggregation was evaluated using dynamic light scattering (DLS) and HPLC technique SEC coupled to static light scattering (SEC-MALS). The influence of stress conditions to the in vitro biological activity was also evaluated. Differences in stability were observed at accelerated stability conditions, 25 °C and 40 °C. According to RPC and CEC the formation of degradation products was slower for both PEGprotein variants and SEC indicates slower formation of aggregates in PEGylated molecules. A combination of PEGylation and the addition of trehalose proved to ensure an even greater stabilizing effect. DLS in average detected a larger number of different size populations in the Protein_t samples than in the PEGprotein samples, which also indicates a higher stability of PEGprotein samples. The research confirmed our initial hypothesis, PEGylation plays a protective role in the stability of the very hydrophobic protein in the liquid pharmaceutical formulation. Both Protein_t and PEGprotein samples retained their in vitro biological activities after exposure to temperature stress.