We have investigated the configuration of immunoglobulin (Ig) genes in leukaemic cells in 17 patients with B-cell leukaemias (11 chronic lymphocytic leukaemias (B-CLL); 4 prolymphocytic leukaemias (B-PLL), and two hairy cell leukaemias (HCL)). In addition we studied four patients with T chronic lymphocytic leukaemia (T-CLL); four patients with acute leukaemia (3 acute lymphoblastic leukaemias (ALL), and 1 mixed acute leukaemia (M.AL)); and six patients with chronic granulocytic leukaemias in blastic crisis (CGL.BC). The heavy chain genes (H) were analysed by using probes for the constant region of the mu chains (C mu) and for the joining region (JH). The light chain genes were analysed by using probes for the constant region of the kappa (C kappa) and lambda (C lambda) chains. We have found rearranged Ig genes in all cases of B-CLL, B-PLL and HCL, but in none of the patients with T-CLL. In one case of HCL, both mu genes were deleted, indicating that in this case the class switch has taken place. In four out of six cases with either ALL or lymphoid CGL.BC and in one case of M.AL, an Ig gene rearrangement was also found. No rearrangement was detected in two cases of myeloid CGL.BC. When the combination of rearrangement versus germ-line configuration was considered, a variety of patterns emerge, but in no case did we find a L chain gene rearranged without at least one H chain gene being rearranged as well. Whereas in the majority of cases of B-CLL only one H chain gene is rearranged, in nearly all cases of B-PLL both H chain genes are rearranged. By systematic analysis of restriction fragment sizes of rearranged genes, we have established that a large number of different variable regions for the H chain (VH) are involved in Ig gene rearrangement in B-cell malignancies. Our data confirm that testing for Ig gene rearrangement may be the most sensitive and specific test for identifying leukaemic cells of B lineage.