In order to estimate the clonality of B-cell lymphoproliferative disorders, polymerase chain reaction (PCR) was used to detect the Ig heavy chain gene rearrangement in DNA extracted from formalin-fixed and paraffin-embedded bone marrow biopsies. 16 out of 19 cases (84%) of B-CLL (n = 9), multiple myeloma (n = 7) and B-non-Hodgkin's lymphoma (n = 3) showed sharp monoclonal bands, while polyclonal smears or no PCR-products were observed in non-neoplastic bone marrow or AML specimens. The results showed that the DNA from paraffin-embedded bone marrow biopsies could be used to detect monoclonal IgH rearrangement by PCR method. This forms the basis for the studies of minimal residual disease or B-ML bone marrow infiltrated diseases.