Results with a dual assay, for the induction of Thy-1+ T cells and of CR+ B cells from marker-negative precursors, confirm that thymopoietin is at present the only known selective inducer of prothymocytes. In contrast, various inducers, including ubiquitin, are active in both assays. Pharmacological evidence indicates that there are different cellular receptors for ubiquitin and thymopoietin. Prothymocytes and pro-CR+ B cells compose two distinct populations in bone marrow and spleen; their distribution in density gradients is different, and elimination of either population enriches the other proportionately. There are no noteworthy differences between induction of these two populations in regard to (a) kinetics, (b) dependence on temperature and protein synthesis, (c) activation by cAMP, and (d) inhibition by cGMP. The opposite inductive effects of cAMP and cGMP were corroborated by the use of pharmacological agents that raise or lower the levels of intracellular cyclic nucleotides. In contrast, a third induction assay, which monitors acquisition of the PC+ surface phenotype, indicates that this differentiative step, the last known for B cells, is initiated by cGMP and inhibited by cAMP. Induction of PC is also inhibited by thymopoietin, signifying that the inductive selectivity of thymopoietin is not due to restriction of its receptors to the T lineage cells. Rather it seems that receptors for thymopoietin occur also on PC-inducible and other B cells, although in this case geared biochemically to inhibition rather than expression of the succeeding gene program. This suggests a role for thymopoietin in the coordinated interregulation of lymphocyte classes, in addition to its better-known function as the thymic inducer of prothymocytes. Present data conform to a general scheme in which the cyclic nucleotides cAMP and cGMP, and agents that affect intracellular levels of these mediators, influence reciprocally the early and late (functional) phases of lymphocyte differentiation as a whole, while thymopoietin influences reciprocally the differentiation of the B and T classes of lymphocyte.