In Hodgkin's disease, the malignant Hodgkin and Reed-Sternberg (HRS) cells are present in very small numbers in the diseased tissue, thus making molecular analysis of these cells very difficult. Using micromanipulation and single-cell polymerase chain reaction (PCR), rearranged immunoglobulin genes can be amplified and sequenced from single HRS cells. Oligonucleotides chosen from the variable (V)-gene sequences identified in the HRS cells can be used as specific markers for the tumour clone. This technique will allow one to search for members of the tumour clone in various compartments of the patient's body, to follow disease progression during therapy, and to analyse stem-cell populations for contamination by tumour cells before autologous bone-marrow transplantation.