Dialysable leukocyte extracts (DLE) are complexes consisting of a large number of low-molecular-weight substances. These extracts have immunomodulatory properties, which are mainly attributed to small peptides commonly referred to as 'transfer factor'. This review focuses on the characteristics of DLE with transfer factor activity, together with the methods for their preparation and purification. The opportunities for applying the extracts for immunotherapy purposes against various diseases in humans and domestic animals are also discussed. Their ease of preparation and relatively low cost combined with the rapid positive effect they produce will make DLE subject to application in medicine in the future, including their use against new nosological entities.