In this paper, we have described the critical experiments leading to the discovery and analysis of the cdc25 M-phase inducer. We have shown that timing of mitosis is sensitive to the level of cdc25+ expression and that the cellular concentration of p80cdc25 increases as cells approach mitosis. From these observations we conclude that, in S. pombe, rate of accumulation of p80cdc25 plays an important role in determining the timing of mitosis. We postulate that under a given set of conditions, a critical level of p80cdc25 activity is required to undergo mitosis. The actual level that is required can vary depending on ploidy, growth rate, nutritional status of the cell, and perhaps other parameters. These signals may be monitored through the weel pathway leading to tyrosyl phosphorylation of p34cdc2. We have shown that p80cdc25 encodes a phosphate that acts by directly dephosphorylating the Tyr-15 residue of p34cdc2. Our studies strongly indicate that this aspect of the mitotic control network is generally conserved among eukaryotes. It is conceivable, however, that the mode of regulation of cdc25 activity may vary from species to species. Clearly, in S. cerevisiae the cdc25+ homolog, MIH1, in contrast to cdc25+, is not rate-limiting for M-phase onset. It will be important to determine whether the level of cdc25+ homologs in other organisms also oscillates during the cell cycle, or whether their activity is controlled by localization or posttranslational mechanisms, such as phosphorylation. Furthermore, our finding of more than one cdc25+ homolog in a single species suggests an additional level of complexity to the control of M-phase onset by cdc25 in higher eukaryotes that will require further investigation.