The ability of TPA to induce stable phenotypic changes that normally serve as markers of differentiation was examined in the four human non-T, non-B cell lines, NALL-1, NALM-16, REH and KM-3. In all four lines, noncytotoxic concentrations of the phorbol ester caused an extensive reduction in the number of cells expressing cALL surface antigen and terminal deoxynucleotidyl transferase. The disappearance of these markers correlated with the loss of cell proliferation. In one of the cell lines, NALL-1, TPA treatment gave rise to a significant increase in Ia-like antigen and antigen T-101, markers which represent more advanced stages of cell maturation. However, surface or cytoplasmic immunoglobins, indicators of mature B cells, were not detectable. Antigen 3A1, specific for myeloid and for T cells, antigen Leu-4, specific for T cells and antigen CM1, specific for monocytes, were also absent. In all cell lines, exposure to TPA resulted in an approximately two-fold increase in acid phosphatase and beta-glucuronidase activity. The emergence of these phenotype changes was not altered upon repeated washing of the TPA-treated cells. These results demonstrate that while TPA is capable of inducing various non-T, non-B cell lines to differentiate to a limited degree, differences exist between the lines in the extent to which they can mature towards the B-cell stage.