Proteolytic fragments of huntingtin (htt) in human lymphoblast cell lines from HD and control cases were compared to those in human HD striatal and cortical brain regions, by western blots with epitope‐specific antibodies. HD lymphoblast cell lines were heterozygous and homozygous for the expanded CAG triplet repeat mutations, which represented adult onset and juvenile HD. Lymphoblasts contained NH₂‐ and COOH‐terminal htt fragments of 20–100 kDa, with many similar htt fragments in HD compared to control lymphoblast cell lines. Detection of htt fragments in a homozygous HD lymphoblast cell line demonstrated proteolysis of mutant htt. It was of interest that adult HD lymphoblasts showed a 63–64 kDa htt fragment detected by the NH₂‐domain antibody, which was not found in controls. In addition, control and HD heterozygous cells showed a common 60–61 kDa band (detected by the NH₂‐domain antibody), which was absent in homozygous HD lymphoblast cells. These results suggest that the 63–64 kDa and 60–61 kDa NH₂‐domain htt fragments may be associated with mutant and normal htt, respectively. In juvenile HD lymphoblasts, the presence of a 66‐kDa, instead of the 63–64 kDa N‐domain htt fragment, may be consistent with the larger polyglutamine expansion of mutant htt in the juvenile case of HD. Lymphoblasts and striatal or cortical regions from HD brains showed similarities and differences in NH₂‐ and COOH‐terminal htt fragments. HD striatum showed elevated levels of 50 and 45 kDa NH₂‐terminal htt fragments [detected with anti(1–17) serum] compared to controls. Cortex from HD and control brains showed similar NH₂‐terminal htt fragments of 50, 43, 40, and 20 kDa; lymphoblasts also showed NH₂‐terminal htt fragments of 50, 43, 40, and 20 kDa. In addition, a 48‐kDa COOH‐terminal htt band was elevated in HD striatum, which was also detected in lymphoblasts. Overall, results demonstrate that mutant and normal htt undergo extensive proteolysis in lymphoblast cell lines, with similarities and differences compared to htt fragments observed in HD striatal and cortical brain regions. These data for in vivo proteolysis of htt are consistent with the observed neurotoxicity of recombinant NH₂‐terminal mutant htt fragments expressed in transgenic mice and in transfected cell lines that may be related to the pathogenesis of HD.