The solubility of monoclonal antibodies (mAb) affects their production and their intravenous administration to patients. In this work, the solubility of a fully glycosylated and a non-glycosylated human mAb expressed in corn was studied by inducing their precipitation by poly(ethylene) glycol (PEG). The experiments were done using PEG 1450 and 8000 at concentrations ranging from 0% to 30% w/w, at different pHs and temperatures. Additional studies were performed in the presence of Griffonia (Bandeiraea) simplicifolia Lectin II, which binds to glycosylated proteins. These studies show that glycosylation increases the solubility of the antibody and that models based on excluded volume principles or on the statistical correlation of solubilities are unable to capture the effect of glycosylation on protein precipitation by PEG. PEG molecular weight controls the onset point of the precipitation curves but glycosylation is the main effect on PEG precipitation efficiency. A two-stages precipitation pattern was observed when a lectin and PEG were added to the glycosylated or non-glycosylated mAbs.