The NH2-terminal amino acid sequence of the heavy chain of the IgG monoclonal cryoimmunoglobulin Ger was determined. Evidence for the glycosylation of the first heavy chain hypervariable region of this protein was found. The inability of the deglycosylated Fab fragment of Ger to inhibit cryoprecipitation provides direct evidence that the presence of an additional sialic acid residue in a heavy chain's first hypervariable region can account for the cryo properties of this protein. This is the first convincing description of a molecular defect that explains the atypical low temperature solubility of a monoclonal cryoimmunoglobulin.