Thirty-six fluids from non-keratinizing jaw cysts have been examined together with autologous sera by immunoelectrophoresis and double diffusion in agar or agrose gels. Except for one cyst fluid which contained electrophoretically homogeneous ("monoclonal") IgG of the kappa type together with free kappa chains, IgG of cyst fluid was electrophoretically heterogeneous. For the most, IgA of cyst fluid migrated more slowly than IgA of serum, whereas the IgM migrated similarly. The three immunoglobulins showed reactions of antigenic identity with the corresponding Ig classes of serum when examined with rabbit antisera against human IgG, IgA, and IgM. Fluid from the median palatine cyst contained secretory component, which showed a reaction of identity with free secretory component isolated from human saliva, and probably also IgA of the secretory type. Two cyst fluids also precipitated a component in rabbit serum.