Genomic DNA extracted from heavily infiltrated spleens of 5 patients with hairy cell leukaemia was hybridized with genomic probes for immunoglobulin genes. The rearrangement pattern, correlating well with the immunohistochemical findings, was that of a clonal B-cell proliferation in all cases. Analysis of the lambda light chain genome with two different probes enabled more accurate localization of the clonal rearrangements within this genome. A small additional T-cell population identified in 3 cases was immunogenotypic of polyclonal (reactive) nature.