Here we review techniques useful in eliminating or reducing interferences caused by molecules that cross-react in immunoassays. The biochemical rationale for using these techniques is discussed. Examples are taken from recent studies aimed at reducing interferences caused by endogenous molecules such as digoxin-like immunoreactive factors or steroid hormones. In this context the role of protein-binding of cross-reacting molecules is also considered. Immunoassay ligand selectivity can be inherently limited by the heterogeneity of the antigenic response or by the structural similarity of epitopes on multiple ligands. Certain empirical approaches have proved useful in maximizing the analytical specificity of immunoassays. These approaches include isolating the relevant ligands before immunoassay, adjusting the kinetic or equilibrium conditions used during the assays, and developing more specific antisera. The physicochemical properties of the cross-reacting molecule best dictate which technique(s) to use. The approaches discussed here are general and apply to minimizing interference caused by a wide variety of both endogenous and exogenous cross-reacting molecules.