Thymocytes of leukemic AKR mice were analysed with a fluorescence-activated cell sorter (FACS) using monoclonal antibodies (MAbs) reactive with H-2Kk and H-2Dk and conventional hetero-antibodies against MuLV gp-70 antigens. Comparison was made with thymocytes of the low-leukemia H-2k strain C3H, as well as with thymocytes of young AKR mice, late preleukemic AKR mice (6-9 months old) and mitogen-stimulated thymocytes of young AKR mice. Expression of H-2 antigens increased on cells of the majority of tumors, on thymocytes of some late preleukemic mice and on mitogen-stimulated thymocytes. An increase in H-2K and H-2D antigens was noted: imbalance of the K/D ratio in favor of H-2D was observed mostly in tumors with relatively lower total amounts of H-2K and D antigens; ratio shifts in favor of H-2K were also found, mostly in tumors with relatively higher amounts of H-2K and D antigens. Increased expression of MuLV antigens was found on cells of all tumors and on thymocytes of several late preleukemic mice. We show that in primary spontaneous AKR leukemias, in spite of large individual differences, the expression of high amounts of MuLV gp70 is not random, but is associated with high expression of H-2K and H-2D antigens. The same phenomenon is seen in thymocytes of preleukemic mice, but in mitogen-stimulated thymus cells increase of H-2 expression is not accompanied by increase of MuLV gp70.