By using a panel of monoclonal antibodies reactive with lymphoid differentiation antigens and microfluorometry, we investigated the malignant cells from five patients with T-cell lymphoblastic lymphoma both at initial involvement and at relapse. We have observed changes in cell surface antigens densities in four of the five cases. These changes could be interpreted as a shift in the status of differentiation of the malignant cells. In two cases the malignant cells appeared to be arrested at an earlier stage at relapse than at initial involvement, while in the two other cases, cells at relapse appeared to be arrested at a later stage than at initial involvement. Interestingly, a parallelism was observed between regression of the differentiative status of the malignant T cells and “leukemic conversion.” These observations are also relevant to the understanding of the development of tumor resistance to chemotherapy.