Phosphoprotein patterns in two mutants of Saccharomyces cerevisiae, cdc25-20(ts) and cdc25-20(ts) bcy1, were analysed by two-dimensional polyacrylamide gel electrophoresis. Comparison with the phosphoprotein patterns of the mutants cyr1-2(ts) and bcy1, analysed in a previous study, demonstrated not only that the CDC25 gene product is a positive element in the regulation of adenylyl cyclase activity, as suggested by recent studies, but that it is also a negative element in the phosphorylation of a 31 kDa protein (p31c and p31d), a protein whose phosphorylation is correlated with cell cycle arrest, and dephosphorylation with cell cycle initiation, respectively. Moreover, the phosphorylation phenotype of p31c and p31d suggests that the activity of the CDC25 protein is subject to feedback regulation by cAMP-dependent protein kinase, and that the CDC25 protein is a key element in an ammonium (NH+4) signal-response system.