The synthesis of a factor, PF, necessary for chromosome pulverization in Sendai virus-induced fused cells was studied in a Chinese hamster cell line (Don). Puromycin (PM), cycloheximide (CHE), and actinomycin D (AMD) were used to block protein and RNA synthesis, especially in the G2 period, including the time during which PF synthesis is expected to take place. Terminal points of action of the drugs on protein and RNA synthesis required for G2-metaphase transition were first determined, and those used for the Don cells were 125 ± 10 and 65 ± 5 minutes before the onset of meta phase, respectively. Substantial numbers of metaphase cells produced in the presence of either PM or CHE for 65 minutes before fusion failed to induce pulverization when made to coexist with interphase nuclei in multinucleate cells, as a result of subsequent cell fusion by Sendai virus. Cells produced in the presence of AMD for 125 minutes could still induce pulverization in multinucleate cells. These findings indicate that PF formation is related to protein synthesis in late G2, and that if RNA is necessary for PF formation, its synthesis probably takes place before G2. Detailed analysis of pulverization frequency as a function of time of exposure to inhibitors, before fusion, revealed that synthesis of PF takes place from 15–45 minutes before the onset of metaphase. Furthermore, PF synthesis in G2 is independent of virus-dependent metabolic events related to protein synthesis. PF may not be necessary for G2-metaphase transition and its exact role in normal cell physiology remains unknown.