In mammals, oocytes are maintained at the G2/M border of the first meiotic division, often for years, before being ovulated. The oocytes wait for a signal that will trigger the chromosome segregation necessary to produce a haploid cell, which is required for fertilization. This event, meiotic resumption, is determined by activation of the M phase-promoting factor (MPF), constituted of the p34cdc2 kinase (p34) and cyclin B. MPF activation in oocytes of large mammals requires protein synthesis. The accumulation of an initiator protein to a critical level plays a key role in determining the cell-cycle timing of p34 kinase activation. In this study, we undertook a search to identify the initiator protein that controls meiosis in the cow. Oocytes were cultured in various conditions before being processed for two-dimensional (2D) gels or Western blotting or fixed for nuclear evaluation. Comparison of 2D patterns of the synthesized protein required for meiotic resumption suggest an initiator role for cyclin. Immunodetection of other proteins that may be initiator proteins (p34, cdc25, and microtubule-associated protein kinases) in immature oocytes, and the absence of cyclin in these oocytes, suggest an important role for cyclin B. Human cyclin B1 protein microinjected into cycloheximide-treated bovine oocytes triggered meiotic resumption. We can conclude that the quantity of cyclin present in bovine oocytes is critical to meiotic resumption.