Chronic lymphocytic leukaemia (CLL) is a malignancy of CD5(+) B cells. This B-cell lineage is established during ontogeny and replenished by the process of self-renewal. Spontaneous and induced leukaemias that frequently affect this lineage are thought to arise as a result of the frequent cell division required to maintain the population throughout adulthood and in response to repeated exposure to environmental antigens. In a series of bone marrow transplant (BMT) experiments performed in B6D2F1 mice, B-cell leukaemia occurred in recipients of serially transplanted syngeneic bone marrow. This study was therefore designed to determine the frequency and phenotype of the observed leukaemia. Male donor cells were initially transplanted into lethally irradiated female hosts and secondary (2 degrees ) BMT was performed at 3 months. At 1, 2, 3 and 16 months following primary (1 degrees ) BMT, and when 2 degrees BMT recipients developed leukaemia, animals were sacrificed and their tissues extensively examined. These analyses confirmed a host-derived CD5(+) transplantable B-cell leukaemia that was initiated in 50% of 1 degrees BMT recipients. With serial passage, the leukaemia became more aggressive and lost CD5 expression, suggesting transformation to a high-grade leukaemia/lymphoma. This previously unreported observation suggests that the combination of radiation and subsequent serial transplantation induces a proliferative stress to the host B-cell compartment that is causative in leukaemic transformation.