DNA polymerase beta levels were measured in 4 cell lines of normal human skin fibroblasts and in 5 cell lines of skin fibroblasts from patients with ataxia telangiectasia, an autosomal recessive disease exhibiting marked X-ray sensitivity. The enzyme specific activities for the normal lines were similar and the mean value was 2-fold lower than the mean value for the ataxia lines. With both kinds of cells, the enzyme level did not change as the cultures progressed from logarithmic to stationary phase of growth. Thus, this putative DNA repair enzyme appears to be 'constitutive' in human skin fibroblast lines, and a modest elevation of beta-polymerase activity is associated with ataxia telangiectasia. These results are discussed in the context to current views about DNA-repair enzymes in X-ray-sensitive cultured mammalian cells.