EL4 tumour cells maintained in culture were separated by FACS analysis to Ly-6.2 negative and Ly-6.2 positive subsets. The Ly-6.2 negative subset gained expression of this determinant on repeated in vivo passage in C57BL/6 mice. Both subsets injected intraperitoneally or intramuscularly in syngeneic mice induced identical changes in lymphocyte profiles. There was generalised lymphocytolysis in both T- and B-cell compartments. The Lyt-1+, 2- T-lymphocytes were more susceptible to cytolysis causing an alteration of the proportional representation of the Lyt-2+ subset from 30% of splenic T-cells (in normal mice) to over 90% of remaining T-lymphocytes in tumour bearing mice. There was thymic regression in both groups of mice with a resultant thymocyte population expressing the range of phenotypes of mature medullary cells. In spite of similar rates of growth both in vivo and in vitro and identical effects on the lymphoid system the Ly-6.2 negative and Ly-6.2 positive tumour subsets were different in their metastatic potential. Mice injected intramuscularly with either subset had enlarged spleens by the second week of tumour growth caused largely by the accumulation of Ig, Lyt-1 and Thy-1 negative cells. Tumour cells were present only in the group injected with the Ly-6.2+ subset. These mice died of their tumour load a week earlier than those injected with the Ly-6.2- tumour cells.