The peripheral mononuclear cells from 23 normal individuals and the purified B cells from 38 patients with chronic lymphocytic leukemia (CLL) were examined by Fourier transform infrared (FT-IR) spectroscopy. Differences were observed between the CLL and normal cells at the DNA, protein and lipid levels, with CLL cells having greater DNA and lower lipid contents than normal cells. In addition, the spectral character of the CLL and normal cells varied, demonstrating that there were also qualitative differences in the DNA and lipids. Statistical analysis, based on hierarchical clustering, separated normal from CLL cells completely and classified them into two subgroups for normal cells, while the CLL cells could be divided into three subgroups that were distinct from the normal cells. These differences were based on the lipid and DNA content and the overall spectral character of the cells.