CD45 isoform expression was studied in 204 cases of B-cell lymphoproliferative disease, including 162 chronic lymphocytic leukaemia (CLL). In almost half the samples tested, CD45R0 was co-expressed with CD45RA, unlike normal B-cells, which express only CD45RA, except at terminal stages of differentiation. In a small number of cases CD45R0 was the dominant isoform expressed. No correlation could be discerned either with Binet or RAI staging in CLL or with disease type CLL, prolymphocytic leukaemia (PLL) or hairy cell leukaemia (HCL). Comparison of CD45 isoform expression with other markers showed no correlation with apparent maturational status of the cells involved.