We developed previously a resistant cell line, CEM/C2, from the human leukemia cell line CCRF-CEM by stepwise selection in camptothecin. This cell line is 974-fold more resistant to camptothecin than parental cells. Resistance is only partially explained by 2-fold reductions in topoisomerase I protein and mRNA levels. We further investigated biochemical and molecular features of topoisomerase I in the resistant cell line. Sequence analyses of the top1 cDNA from CEM/C2 identified mutations corresponding to two amino acid substitutions, Met370Thr and Asn722Ser. Asn722Ser is next to the catalytic Tyr723 in a region highly conserved among type I eukaryotic DNA topoisomerases. Recombinant top1 with the corresponding substitution was found to be catalytically active and resistant to camptothecin. These results indicate that camptothecin resistance of CEM/C2 is due to the mutation Asn722Ser and strongly suggest that the asparagine immediately flanking the catalytic tyrosine is important for the camptothecin action.