Immunoglobulin heavy chain (IgH) variable region (VH) genes are rearranged and expressed in a programmed manner during B-cell development. In common with foetal/pre-immune B-cells, malignant B-lymphoid populations preferentially use a restricted repertoire of developmentally regulated VH genes. By nucleotide sequence analysis of polymerase chain reaction amplified IgH genes, we have compared the repertoire of VH1 family genes that are rearranged in mature, CD5+ B chronic lymphocytic leukaemia (CLL) with that in immature, CD5-B-lineage acute lymphoblastic leukaemia (ALL). The results revealed a non-random pattern pf VH1 usage in which no single VH1 family member was common to each of these disease groups. The VH1 gene, 51P1, which underlies an auto-antibody associated cross-reactive idiotype, 'G6', frequently expressed on foetal B-cells, was preferentially rearranged in CLL (three of nine rearranged alleles). Another developmentally regulated VH1 gene, 20P3, accounted for more than half of the VH1 specific IgH gene rearrangements in ALL (five of nine VH1 alleles). Such developmentally restricted VH1 genes may distinguish discrete, although not necessarily exclusive, stages or compartments in B-lymphopoiesis from which each of these disease types arise.