Lysozyme dimer is the active ingredient of Lydium-KLP (Nika Health Products), an immunomodulating drug used in veterinary practice. It is a highly purified semi-synthetic substance, obtained by the chemical polymerization of natural N-acethylmuramylohydrolase found in the albumen of chicken eggs. Preclinical trials in vitro have revealed that lysozyme dimer, depending on its concentration, is able to enhance the production of IFN- and modulate the synthesis and release of IL-2, interleukin-6 (IL-6) and TNF- by cultured human lymphocytes stimulated by concanavalin A (Con-A). The in vitro trials show that lysozyme dimer stimulates the phagocytic activity of leukocytes isolated from cow’s milk and blood. Our preclinical in vivo studies investigated the dose-dependent immunotropic effects of lysozyme dimer using various experimental models. Immunomodulating or immunocorrecting effects of lysozyme dimer were defined on inbred strain of mice stimulated with thymus-dependent antigen, i.e. sheep erythrocytes (SRBC), or pharmacologically suppressed by the administration of high dose of cyclophosphamide (200 mg/kg) or hydrocortisone (125 mg/kg), or subjected to acute stress (immobilization stress). The article presents the current state of knowledge on the immunomodulatory properties of lysozyme dimer under conditions of stimulation or suppression of the immune system. We review what is known specifically from preclinical trials about the adjuvant action of lysozyme dimer on the primary and secondary humoral response of SRBC-immunized mice as well as about its immunocorrective effect.