Certain rare human diseases with autosomal recessive mode of inheritance are associated with a greatly increased cancer frequency which may reflect specific defects in DNA repair or replication. These disorders include xeroderma pigmentosum, ataxia-telangiectasia, Fanconi's anaemia and Bloom's syndrome. Cells from individuals with Bloom's syndrome usually grow slowly in culture and exhibit increased chromosomal breakage and rearrangement, an elevated frequency of sister chromatid exchanges, retarded rates of progression of DNA replication forks, delayed conversion of replication intermediates to high-molecular-weight DNA, and slightly increased sensitivity to DNA-damaging agents. Several of these features are also characteristic of Escherichia coli and yeast mutants with a defective DNA ligase. In this investigation we show that one of the two DNA ligases of human cells, ligase I, is defective in a representative lymphoid cell line of Bloom's syndrome origin.