Ionized calcium plays a major role in the regulation of cellular processes in both eukaryotic and prokaryotic cells (1). A wide variety of cell surface receptors and ion channels utilize a calcium signal to initiate events such as cell motility, contraction, and secretion. To a large extent, the advent of fluorescent indicators of free calcium ion concentration that could be loaded into cells in a nondisruptive manner has been responsible for our current knowledge of cellular calcium homeostasis (2). Typically, the fluorescent signal has been monitored using cuvet-based fluorometers or by confocal fluorescent microscopy. Although acceptable for a number of applications, these methods are relatively labor intensive and are not suitable for screening large numbers of compounds.