Chitosan oligomers are known elicitors of plant defence mechanisms. In this work, chitooligosaccharides of different degrees of polymerization and degrees of acetylation were prepared and characterized by matrix-assisted laser desorption ionization time-of-flight mass spectrometry. The effect of the degree of polymerization (DP), degree of acetylation and concentration of these chitooligosaccharides on defence activation in Arabidopsis thaliana suspension-cultured cells was studied. Our study results show that fully deacetylated chitooligosaccharides (chitosan oligomers) induce, depending on their DP and concentration, phenylalanine ammonia-lyase (PAL) activation, HO synthesis and cell death in A. thaliana cell suspensions. The progressive reacetylation of the chitosan oligomer elicitors progressively impaired their ability to enhance HO accumulation and cell death, but did not affect the activation of PAL.