ICs were used in a nested competitive PCR assay to quantitate rearranged NHL cells. (A) The following number of copies of IC (GU1M): (1) 5,000; (2) 3,200; (3) 1,600; (4) 800; (5) 500; (6) 320; (7) 160; (8) 50; (9) 0 were added to aliquots of H2 cells mixed with normal PBMC at a ratio of 5 × 10³/10⁶. Each aliquot, a positive (10: IC only) and a negative control (11: water only) were amplified by nested PCR, run on a 2.5% agarose gel, and stained with ethidium bromide. (B) Amounts of PCR products were measured by multidensitometric analysis. (C) Fluorescence ratios (•) were plotted against the concentration of IC (corrected for differences in molecular weight). The estimated number of bcl-2/IgH rearranged cells in the sample corresponded to a ratio of 1 (correlation coefficient = −.945).