To replicate, the trypomastigote form of Trypanosoma cruzi must invade host cells. Since a role for Ca2+ in the process of cell invasion by several intracellular parasites has been postulated, changes in the intracellular Ca2+ concentration in T. cruzi trypomastigotes and in tissue culture L6E9 myoblasts during their interaction were studied at the single cell level using digital imaging fluorescence microscopy or in cell suspensions by fluorescence spectrophotometry. An increase in cytosolic Ca2+ in T. cruzi trypomastigotes was detected at the single cell level after association of the parasites with the myoblasts. Ca2+ mobilization in the host cells was also detected upon contact with trypomastigotes either at the single cell level or in cells grown in coverslips and exposed to suspensions of trypomastigotes. Pretreatment of the parasites with the Ca2+ chelators quin 2 (50 microM) or bis-(o-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (BAPTA, 50 microM) decreased the trypomastigotes' association to myoblasts by approximately 40 and 63%, respectively, thus indicating that an increase in intracellular Ca2+ concentration in the parasites is required for cell invasion in addition to Ca2+ mobilization in the host cells.