The phospholipid and ganglioside composition in bone marrow progenitors of lymphocytes, thymocytes and mature lymphocytes of intact rats and rats with sarcoma 45 were studied. The lymphocytes and their progenitors were isolated by Ficoll-Paque density centrifugation. The phospholipids and gangliosides were separated by thin-layer chromatography following standard chloroform:methanol extraction from the cells. Alterations in the lipid spectrum (both phospholipids and gangliosides) were shown to take place during lymphocyte differentiation. The rate of ganglioside sialylation diminished, which was expressed as an increase in mono- and di-, and a decrease in tri- and tetrasialoganglioside levels. Tumor-induced alterations in lymphocyte lipid composition involve all stages of lymphocyte differentiation. These shifts are believed to be connected with a disturbance of the antineoplastic function of lymphocytes and, consequently, the immune response of the tumor-bearing organism.