Stromal cells appear to be key regulatory elements in hematopoiesis and lymphopoiesis. Several stromal cell lines can support B lineage by creating a hematopoietic microenvironment via cell contact or regulatory humoral molecules. These activities have been efficiently mediated by an adipocytic stromal cell line 14F1.1 on infant leukemia cells expressing a hybrid pre-B myeloid phenotype. Several murine cell clones, however, are known to have different ability to support growth and/or differentiation of leukemic cells depending on the maturational stage in which malignant cells are frozen. Pre-B-cell lines and fresh leukemias were therefore cultivated on S17 stromal cell line, before and after exposure to human recombinant interleukin 4 (rIL4), a cytokine whose effects on the growth and differentiation of the B-cell compartment depend on the developmental stage of the target B cell. In the present work, leukemic cells, both in suspension and in close contact with stromal cells, maintained their original phenotype throughout the whole period of co-culture with S17, either before or after exposure lo human rIL4.