Lydium KLP (dimer lysozyme) used in veterinary practice as an immunostimulator has different effects of action on humoral immunity of rabbits compared with endotoxin (LPS) activity. E. coli endotoxin (Kroeger O8) given intravenously induced humoral immunity in rabbits after its first injection while the first administration of Lydium KLP hardly stimulated humoral immunity in comparison to the second dose. LPS is commonly regarded as a pirogen while Lydium KLP is not. The results obtained indicate different effects of Lydium KLP and LPS on antibody production and on temperature release.