An increased level of cytoplasmic free ionized calcium [Ca2+]i after crosslinking of membrane receptors is a critical second messenger in the activation of T and B lymphocytes. The availability of fluorescent calcium chelators, such as quin-2 and indo-1, makes accurate measurement of [Ca2+]i possible. One of the major drawbacks of spectrofluorometry which is the generally used method in such studies is that the overall response of a cell suspension is recorded. Such data will be biased by the proportion of non-responding cells, which will differ according to the purity of cell populations and the nature of the stimulus applied. An accurate and reliable technique to measure intracellular free calcium responses in indo-1-loaded cells at the single cell level has been developed using a simple mercury arc lamp-based flow cytometer, the FACS analyzer. Using this technique we have found that the rapid increase in [Ca2+]i (within 30 s) in T cells following activation by ConA involves a minority of cells, whereas all T cells show increased [Ca2+]i levels within 2-3 min.