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We have studied how the cell cycle‐specific oscillations of mitotic B‐type cyclins are generated in mouse fibroblasts. A reporter enzyme comprising the N‐terminus of a B‐type cyclin fused to bacterial chloramphenicol acetyl transferase (CAT) was degraded at the end of mitosis like endogenous cyclins. Point mutations in the destruction box of this construct completely abolished its mitotic instability. When the destructible reporter was driven by the cyclin B2 promoter, CAT activity mimicked the oscillations in the level of the endogenous cyclin B2. These oscillations were largely conserved when the reporter was transcribed constitutively from the SV40 promoter. Pulse‐chase experiments or addition of the proteasome inhibitors lactacystin and ALLN showed that cyclin synthesis continued after the end of mitosis. The destruction box‐specific degradation of cyclins normally ceases at the onset of S phase, and is active in …