A method for the detection of immunoglobulin light chain mRNA by situ hybridization has been applied to routine bone marrow trephines, from patients suspected of having multiple myeloma. The principle aim was to define monoclonal populations even in cases lacking a paraprotein or an obvious atypical plasma cell proliferation. The expression of light chain mRNA in plasma cells in routine bone marrow trephines from 14 patients with suspected myeloma was studied. Plasma cells in the marrow ranged from 4% to 99%. Clonal populations based on light chain restriction were delineated in 12 patients, using biotinylated probes to kappa and lambda mRNA and visualized using an alkaline phosphatase/fast red naphthol capture method. Normal tonsil and bone marrow trephines were used as controls. In situ hybridization was found to be a specific, safe and rapid technique. It could be applied to both fixed and fresh tissue. It was found to lack the background staining of interstitial immunoglobulin, particularly in sclerotic cases, which is so often encountered with immunocytochemistry. Additionally, this technique will type plasma cell neoplasms (undetected by routine immunocytochemistry) in post-transcriptional block or non-secretory myeloma.