The volume change accompanying the Quellung reaction of Type I pneumococci has been studied by two methods: 1) direct optical measurement, with the microscope, of a large number of cells, and calculation of volumes from average dimensions, considering each cell a prolate spheroid; and 2), counting the number of cells and measuring their total volume after packing to a constant level in the capillary base of specially calibrated Hopkins' tubes, at centrifugal forces up to 142,000 times gravity.

Before Quellung, the average volume per cell was found to range from 1.26 to 3.87 cubic microns, including results of both methods. The ratio of the volume of swollen: unswollen cells by the two methods agreed within better than 10 per cent. The volume increase accompanying the Quellung varies with the potency of the antiserum, and the cells used in different experiments, ranging from approximately 3 to 8 times the volume previous to the Quellung. The average increase in volume per cell in homologous antiserum diluted 1:3, amounted to from 2.9 to 10.1 cubic microns. The centrifugal method has an accuracy altogether satisfactory and should provide a convenient means for investigating the volume change accompanying Quellung in relation to various factors.

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