P34(cdc2) is a key cell-cycle protein in fission yeast that is necessary for progress in the cell cycle from the G1 to the S phase and from G2 through mitosis. Homologues of p34(cdc2) have been found in all eukaryotes that have been investigated. Levels of p34(cdc2)-like protein were studied by quantitative Western blotting in developing cotyledons of Daucus carota L. (carrot) seedlings, in expiants from the same seedlings transferred to tissueculture media with and without 2,4-dichlorophenoxyacetic acid (2,4-D), and in nutrient-starved suspension cultures derived from carrot callus. During the cessation of cell division, which accompanies development of the cotyledon to maturity, there was a 16-fold decline in the level of the p34(cdc2)-like protein. Auxin-stimulated dedifferentiation in excised tissue from mature cotyledons was accompanied by restoration of the level of p34(cdc2)-like protein, and the responding cells formed a callus. These data support our earlier proposition, based upon evidence from wheat leaf, that changes in the level of p34(cdc2)-like protein act in the switch between cycling and differentiation. Persisting high levels of p34(cdc2)-like protein in suspension cultures, when division was stopped by nutrient limitation, indicated that decline of the protein was not an inevitable consequence of the cessation of division. Decline of p34(cdc2) in differentiation may therefore be a regulated process that determines exit from the cell cycle and the converse increase in p34(cdc2) may be a regulated process controlling dedifferentiation and resumption of cell division. 