To investigate the mechanisms whereby treatment with chitosan (CHN) is observed to increase the capacity of plants to resist pathogens, CHNs of different molecular weights (MWs) prepared by enzyme hydrolysis were used to treat rice cells in suspension culture and also rice seedlings. The results obtained with cultured cells showed that in this material CHN treatment could trigger a set of defence responses, including the production of hydrogen peroxide (H2O2), increases in the activities of phenylalanine ammonialyase (PAL; EC 4.3.1.5) and chitinase (CHI; EC 3.2.1.14), increases in transcription of defence-related genes beta-1,3-glucanase (glu) and chitinase (chi) and accumulation of pathogen-related protein (PR1). Furthermore, CHNs of different MWs were observed to have different capacities to induce defence responses. CHNs of low MWs were more effective at inducing the described defence responses than those of higher MWs. Enhanced defence against rice blast pathogen Magnaporthe grisea 97-23-2D1 was observed in rice seedlings treated with low MW CHNs compared to seedlings treated with higher MW CHNs. In all cases, suppressing the production of H2O2 by adding scavengers dimethylthiourea (DMTU), 2,5-dihydroxycinnamic acid methyl ester (DHC), catalase (Cat) or ascorbate (As) blocked the defence responses. These results indicate that CHNs of low MWs have a greater capacity to induce the production of H2O2, thus resulting in stronger defence responses, than those with higher MWs.