Morphometric study of the erythroid stem was performed in aggressive and indolent non-Hodgkin's lymphomas vs. other cells and tissues of the bone marrow (including the tumor tissue) before chemotherapy. Hypoplasia and abnormal maturation of the erythroid stem were particularly pronounced in diffuse infiltration of the bone marrow, which did not depend on lymphoma aggressiveness. Hypoplasia of the erythroid stem was often observed during focal infiltration of the bone marrow with lymphoma cells (despite a smaller area of tumor tissue in aggressive lymphomas than in indolent lymphomas). A decrease in the relative area of adipose tissue, smooth resorption of bone tissue, and myelofibrosis are the major changes in the bone marrow microenvironment.