Two IL 3-dependent bone marrow-derived cell lines, FD.C/1 and 32Dcl-23, have been converted to IL 2-dependent growth states. In the case of the FD.C/1 cell line, a small subpopulation of cells converted to IL 2 growth dependence, whereas for the 32Dcl-23 cell line, most cells readily converted to IL 2 growth dependence. IL 2 receptor expression was followed in FD.C/1 and 32Dcl-23 cells in both IL 3-and IL 2-dependent growth states, by analyzing transcription of IL 2 receptor-specific RNA and the subsequent expression of cell surface receptors. The exposure of IL 3-dependent cell lines to IL 2 resulted in rapid increases in the transcription of IL 2 receptor-specific RNA and receptor expression. The addition of IL 3 to IL 2-dependent cell lines did not alter the expression of the IL 2 receptor. When IL 2 was removed from the culture medium of these cells, the presence of IL 3 maintained a higher level of IL 2 receptor expression than was observed in the absence of lymphokines.