A method based upon the principle that unlike domains of bonding are reflected in different reactivities and distribution of residues that can be crosslinked, has been elaborated for the determination of symmetry of oligomeric proteins. The derivation of theoretical curves for the prediction of crosslinking patterns of tetramers produced by reaction with a bifunctional reagent and subsequent sodium-dodecylsulphage-gel electrophoretic analysis, is presented. Based upon the theory the symmetry properties of a tetramer, to the extent whether it is an isologous or heterologous association, can be deduced by a simple calculation. Crosslinking patterns obtained with rabbit muscle aldolase and pig muscle lactate dehydrogenase after treatment with a series of diimidoesters of increasing chain length are evaluated and shown to be consistent with the expectations for isologous tetramers. From the patterns obtained with the various reagents the distances between lysyl residues located nearest to each other in different subunits in the two proteins could also be determined.