Plasmodiophora brassicae causes clubroot in the turnip, Brassica rapa L. We used organ cultures of adventitious roots from B. rapa seedlings to investigate the initial response of resistant and susceptible cultivars to P. brassicae infection. Primary plasmodia of P. brassicae were observed in root hairs of both susceptible and resistant cultured roots. On the other hand, secondary plasmodia were able to proliferate only in the susceptible root culture but not in the resistant one. Root cultures from the susceptible cultivar all developed clubroot 4 weeks after treatment with 10(4), 10(5) or 10(6) spores/ml, but roots from the resistant cultivar did not develop clubroot under the same conditions. Cell death, as measured by Evans blue and TTC dye methods, was observed in cultured roots from the resistant cultivar but did not occur in roots from the susceptible cultivar after exposure to P. brassicae spores. Cell death was inhibited almost completely by EGTA and verapamil but not by the calmodulin antagonist W7. These results suggest the involvement of Ca2+ in P. brassicae-induced cell death. Alkalization of the root culture medium of the resistant cultivar was observed 2 days after treatment with P. brassicae spores but was not observed in root culture medium from the susceptible strain. We conclude that our root culture system must be a useful tool for further studies of the molecular mechanism of clubroot resistance.