Synchronized V79 cells were treated before entering mitosis with known and suspicious mitotic arrestants and analyzed by flow cytometry and by light microscopy. Colcemid, nocodazole, vinblastine, diethylstilbestrol, triethyl lead and cadmium sulfate caused a dose dependent mitotic arrest of up to 80%, in comparison with 6% for the controls. Mixtures of polycyclic aromatic hydrocarbons and heterocyclic compounds induced a mitotic arrest of 50%-60%. Extracts of airborne particulates revealed a mitotic arrest of 10%-40%. In contrast, benzoquinone and hydroquinone led to a G2-block rather than to a mitotic arrest. Results of flow cytometry measurements correlated well with those obtained by light microscopy. Cell synchronization in combination with flow cytometry seems to be of considerable value as a rapid method for testing nongenotoxic agents with mitotic arresting activity.