Abstract: Polyethylene glycol serum immunoglobulin conjugates exhibit substantial rstance to degradation by intestinal enzyme while retaining their immuno-activity. Thus, PEG-IgG or PEG-IgA conjugates can be used as orally administered therapeutics to treat patients with gastrointestinal immunodeficiency to reconstitute secretory immunity. Preferred conjugates are made by reacting activated PEG and IgG in ratios of from 1:5 to 1:1000 such that less than about 27% of the IgG lysine residues are bonded to the PEG. The conjugates are advantageously formulated into a pharmaceutical composition comprising the conjugate and a pharmaceutically acceptable oral carrier. Particularly for administration to infants, a preferred oral carrier is milk.