The profound depression of fluid pinocytosis observed in mitotic cells (Berlin, R. D., et al. 1978. Cell. 15:327--341) is documented by quantitative microspectrofluorimetry of fluorescein-labeled dextran uptake in single cells. In J774.2 macrophages, fluid pinocytosis is reduced 30-fold during mitosis. The depression develops within 30 s of entry into prophase and recovers with equal rapidity upon emergence from telophase into G1. This characteristic pattern of fluid pinocytosis forms the basis of a new method for detailed kinetic analysis of the duration of mitosis and its phases. The analysis is applied to the J774.2 macrophage cell line but should be generally applicable to other lines. Effects of ouabain and colchicine on the length of mitosis and its phases are evaluated, revealing a selective prolongation of metaphase by ouabain and suggesting a role for microtubules in the transition from G2 into mitosis.

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