Separation of mouse rosette-forming cells (MRFC) was used as a technique to isolate T lymphocytes in patients with chronic lymphocytic leukaemia. The resulting interface fraction (non-MRFC) was highly enriched for sheep rosette-forming T cells (mean value 71.8% SRFC) as compared with unfractionated peripheral blood lymphocytes (mean value 18% SRFC). A higher degree of T-cell purity was achieved by this method than by the sheep rosette sedimentation technique in patients with leucocyte counts greater than 50 X 10(9)/l. Functional tests revealed that depletion of MRFC increased not only phytohaemagglutinin and mixed lymphocyte culture (MLC) reactivity but also the stimulatory capacity in the allogeneic MLC. In contrast, separation of MRFC in normal blood donors gave only a poor degree of separation and was not an effective means to isolate lymphocyte subpopulations.