In order to evaluate whether the SCID mouse can provide the microenvironment for the growth of human immature T-cell leukemias and if in vivo growth alters their phenotype, we examined the behavior of 3 well-characterized T-cell acute lymphoblastic leukemia (T-ALL)-derived T-cell lines which are at different stages of maturation (CEM, SUP-T3 and MOLT-4f) after transfer to non-irradiated SCID mice. All 3 T-cell lines engrafted and proliferated to form tumors in the mice and showed dissemination patterns in the SCID mouse comparable to those of T-ALL in man: i.e., human cells were detectable by flow cytometry or were cultured from mouse bone marrow, spleen, liver or thymus. CEM, which is the most immature T-cell line, readily formed tumors after injection of cells. The more mature T-cell lines, SUP-T3 and MOLT-4f, required a longer time period, even after injection of higher cell numbers. Whereas no changes in the configuration of the rearranged T-cell receptor genes were detected, striking phenotypic changes were observed in all 3 leukemias growing in the SCID mice after injection. SCID-CEM cells showed an increase in the surface expression of CD3 and CD8 and a decrease in the expression of CD1 and CD71 (transferrin-receptor). SCID-MOLT-4f cells showed an increase in CD5 and CD8 expression and a decrease in CD45RA expression. SCID-SUP-T3 cells showed increased expression of CD8 and CD45RA. Apparently, the mouse environment caused changes in cell-surface antigen expression on the T-ALL-derived T-cell lines. Some immunophenotypic changes remained stable during subsequent growth in culture of SUP-T3 cells, suggesting that maturation of the cell line occurred in vivo. The other cell lines, CEM and MOLT-4f after undergoing in vivo-induced changes, reverted to the original immunophenotype, suggesting transitory activation in vivo. These data point out the importance of stromal factors in defining growth and maturation of human leukemic cells in vivo, in SCID mice.