Caffeine had been shown to induce mitotic events in Syrian hamster fibroblast (BHK) cells that were arrested during DNA replication (Schlegel and Pardee, Science 232:1264-1266, 1986). Inhibition of protein synthesis blocked these caffeine-induced events, while inhibition of RNA synthesis showed little effect. We now report that the protein(s) that are required for inducing mitosis in these cells were synthesized shortly after caffeine addition, the activity was very labile in the absence of caffeine, and the activity was lost through an ATP-dependent mechanism. Caffeine dramatically increased the stability of these putative proteins while having no effect on overall protein degradation. Experiments with an inhibitor of RNA synthesis indicated that mitosis-related RNA had accumulated during the suppression of DNA replication, and this RNA was unstable when replication was allowed to resume. These results suggest that the stability of RNA needed for mitosis is regulated by the DNA replicative state of the cell and that caffeine selectively stabilizes the protein product(s) of this RNA. Conditions can therefore be selected that permit mitotic factors to accumulate in cells at inappropriate times in the cell cycle. Two-dimensional gel electrophoresis has demonstrated several protein changes resulting from caffeine treatment; their relevance to mitosis-inducing activity remains to be determined.