The effects of alginate on the physiological activities of plant cells were studied. Addition of alginate oligomer (AO) to the suspension culture of Catharanthus roseus L. or Wasabia japonica cells promoted the production of antibiotic enzymes such as 5＇-phosphodiesterase or chitinase respectively. Ajmalicine (a secondary metabolite) production by C. roseus CP3 cells was also promoted when AO was added to the suspension culture. On the basis of these results, we assumed that alginate is an elicitor-like substance. We therefore compared the effect of AO on C. roseus L. and W. japonica cells with those of chitosan oligomer(CO) and oligogalacturonic acid (OGA), which are well known as an exogenous elicitor and endogenous elicitor respectively. The effects of various concentrations of AO, OGA, and CO on the physiological activities, membrane permeability and protoplast formation of C. roseus L. or W. japonica cells were investigated. AO and OGA showed similar physiological effects, which were quite different from those of CO. Since alginate appeared to have similar effects to galacturonic acid, we concluded that alginate acts as an endogenous elicitor. Both alginate and galacturonic acid are uronic acids, and we considered their structural similarity. The effects of esterification of the carboxylic groups of alginate by propylene oxide were also studied. The greater the degree of esterification, the less the secretion of 5＇-phosphodiesterase. Hence we assumed that carboxylic groups have an important role in the initiation of the elicitation reaction in plant cells, as shown in the case of galacturonic acid.