The prevalent forms of adult and childhood B-cell neoplasia are chronic lymphocytic (CLL) and acute lymphocytic (ALL) leukaemia, and are typified by a nearly monoclonal accumulation of cells expressing a single heavy (H) and light (L) chain variable (V) region. V gene selection could be random, or quite biased if the disease or the developmental status of the transformed cell somehow influenced DNA rearrangement. We have cloned and sequenced three germ-line V_H gene segments that constitute a new human V^H family (subgroup V) linked within 160 kilobase pairs of the D_H-J_H complex. One V_H(V) member is rearranged in about 30% of patients with CLL and ALL, but not in IgM-expressing B-cell lines from peripheral blood. In some tumours, we detect a truncated ( V_H(V) RNA devoid of constant regions that originates from unrearranged V_H(V) genes. In other tumours and in resting splenocytes, we detect large amounts of normally sized V_H(V)-associated mRNA, although stimulation by mitogen of splenic B cells results in loss of V_H(V)-hybridizing RNA. These features suggest that biased rearrangement of subgroup V may be under developmental selection.