Methods for preparations of protein-protein and protein-hapten conjugates are reviewed from the view point of selective formation of the conjugates. The methods are classified into four types depending upon the chemical structures of the modifiers of proteins. The specific methods applied for mono-functional haptens having a carboxylic acid, amino or thiol group and non-specific ones used for polyfunctional modifiers in aids of homo-bifunctional reagents. The selective method utilizing the structural specificities of the protein and also the newly developed selective methods using hetero-bifunctional cross-linkers such as the maleimide succinimidyl ester type are discussed.