The clonal determination of B-cell lymphoproliferative diseases by immunoglobulin heavy chain (IgH) rearrangement by polymerase chain reaction (PCR) is widely used. In this study we report our experiences with B cell clonality analysis using multiplex PCR amplification followed by heteroduplex analysis, which was found to be essential for the efficient resolution of monoclonal bands within polyclonal backgrounds, in formalin-fixed paraffinembedded tissue.