In order to investigate the radioresistance mechanism in human carcinoma cells, we isolated the radioresistant cells from a human KB carcinoma cell line by various methods. Although the radioresistant cells were not isolated by the repeated X-irradiation method, pretreatment of the cells with mutagens including N-methyl-N'nitro-N-nitrosoguanidine and 4-nitroquinoline 1-oxide induced radioresistance of 1.50 to 3.75-fold, as judged by the D0 ratio to parental KB cells. When three representative radioresistant cell clones with stable growth isolated by a different method were examined for cross-resistance to anticancer drugs, each cell clone exhibited resistance to one, five and two anticancer drugs, including mitomycin C, bleomycin and methotrexate, respectively. Among them, the cell line N10, showing stable plating efficiency and population doubling time, was further characterised. Consequently, immunocytochemistry, Western blotting and flow cytometry revealed that N10 expressed a higher level of mutant p53 protein than did parental KB, suggesting the involvement of mutant p53 protein in radioresistance of N10.