It has been proposed that protein-bound oligosaccharides interact with the protein to which they are attached to up- or down-regulate the bioactivity of the 'composite' glycoprotein. Oligosaccharide analyses of the glycoproteins Thy-1, tissue plasminogen activator and immunoglobulin G are presented. Correlations between particular glycoforms and enzymic activities are demonstrated for tissue plasminogen activator. The change in the prevalence of particular immunoglobulin G glycoforms is shown to correlate with disease activity in rheumatoid activity.