A Simple Method for Counting Small Numbers of Cells by Flow Cytometry

We describe a flow cytometric method for the quantitation of small numbers of cells that facilitates proliferation studies in microcultures. A constant number of fluorescent latex microspheres/sample is added to single cell suspensions prepared from the cultures. By flow cytometric analysis, cells are easily distinguishable from the microspheres and can be quantitated on the basis of their light scattering and fluorescence properties is contour plots. As the number of microspheres/sample is known, the relative proportions of cells and microspheres, respectively, can be converted into absolute cell numbers. This quick method is useful for any type of studies where cell numbers less than 1 x 10(5) in a small volume are to be determined.