Within minutes of fertilization, clam embryos begin to make several new proteins, some of which are absolutely essential for progress through the cell cycle and entry into mitosis. Two of these proteins, cyclin A and cyclin B, appear to be regulators of mitosis. They accumulate across the cell cycle and are abruptly destroyed near the end of each mitosis. Direct evidence shows that the rise in cyclin A can induce mitosis; indirect evidence suggests that cyclin B lead to exit from mitosis. The long term goals of this project are to understand the roles of the cyclins in the cell cycles of early embryos and to investigate their functions in somatic cell cycles of vertebrates. (1) Immunofluorescence techniques will be used to visualize the intracellular distributions of the cyclins as cell proceed through the cell cycle. (2) Antibodies and purified cyclin proteins will be used to study the relationship between the cyclins and MPF (M phase Promoting Factor), a key regulator of the cell cycle that has eluded purification and molecular characterizations since its discovery almost twenty years ago. (3) Cell-free systems will be used to identify the specific mitotic processes that are controlled or influenced by the cyclins. (4) Existing and new antibodies and DNA probes will be used to search for cyclin proteins and genes in somatic cells. This work should identify important regulatory steps that control the cell cycle at the G2/M transition and it may reveal new kinds of regulatory mechanisms in somatic cells.