We report the use of capacitance measurements to monitor particle uptake after cellular exposure to phagocytic stimuli. In these studies, human monocyte-derived macrophages (HMDMs) and cells from the murine macrophage-like cell line J774.1 were exposed to immune complexes or sized latex particles (0.8 or 3.2 micron in diameter). An average decrease in cell capacitance of 8 pF was seen after exposure of the cells to immune complexes. Cells in which particle uptake was inhibited by cytochalasin B treatment before exposure to immune complexes showed an average increase of 0.5 pF. The decrease in membrane capacitance after exposure of cells to particulate stimuli was absent with the soluble stimulus, platelet-activating factor, further confirming that decreases in membrane capacitance were due to particle uptake. Exposure of cells to sized latex particles resulted in a graded, stepwise decrease in membrane capacitance. The average step size for 0.8-micron particles was 250 fF, and the average step change for the larger 3.2-micron particles was 480 fF, as calculated from Gaussian fits to the step size amplitude histograms. The predicted step size for the individual particles based upon the minimum amount of membrane required to enclose a particle and a specific capacitance of 10 fF/micron2 was 20 and 320 fF, respectively. The step size for the smaller particles deviates significantly from the predicted size distribution, indicating either a possible lower limit to the size of the phagocytic vacuole or multiple particles taken up within a single phagosome. Dynamic interaction between phagocytosis and exocytosis was observed in a number of cells as a biphasic response consisting of an initial rapid increase in capacitance, consistent with cellular exocytosis, followed by stepwise decreases in capacitance.