Two cases of chronic lymphocytic leukaemia are presented in which, in the terminal phase of the disease, a population of abnormal lymphocytes similar to those seen in lymphosarcoma cell leukaemia replaced the morphologically normal small lymphocytes observed previously. Immunologically, this change coincided with a striking alteration in the pattern of surface immunoglobulin markers. In both cases, most cells initially carried IgM and in both cases these were replaced by cells carrying IgG as the number of abnormal lymphocytes increased. In addition, the use of anti-delta antiserum in the second case revealed the coexistence of large numbers of IgD-bearing lymphocytes as well. Calculations showed that more than half the cells present at that time must have been carrying both IgD and IgG surface markers.