The early biochemical responses stimulated by the action of mitogens and growth factors on mouse thymocytes and 3T3 fibroblasts are analysed as part of a systematic attempt to define the mitogenic pathways from G0 to S phase in these cells. Although the primary response to each mitogen can be distinguished by the pattern of secondary responses they initiate, there is substantial overlap in these responses. The aim is therefore to determine whether there is early convergence on a common mitogenic pathway, defined by a sequence of responses obligatory for progression from G0 to S phase for different mitogens and cell types. The ‘dual-signal’ hypothesis for the mitogenic stimulation of thymocytes is a simple version of a common mitogenic pathway. It proposes that the T-cell receptor initiates the pathway via the breakdown of phosphatidylinositol (4,5)-bisphosphate to generate a Ca signal (from the release of inositol (1,4,5)-trisphosphate) and to activate protein kinase C (from the release of diacylglycerol). The rationale for this hypothesis lies in the co-mitogenic action of the Ca2+-ionophore, A23187, and the phorbol ester, 12-o-tetradecanoyl phorbol 13-acetate, which is assumed to activate specifically protein kinase C. However, detailed analysis of the coupling between some of the early responses, including the Ca and pH signals, phosphatidylinositol (4,5)-bisphosphate metabolism, c-myc gene activation and general metabolic stimulation, indicates clearly that the hypothesis is inadequate to account for the initiation of the normal mitogenic pathway in thymocytes.