(Carruthers and Sibley, 1999). To test whether calciumregulated secretion of micronemes is necessary for cell invasion, we examined the effects of a highly speci®c chelator of intracellular Ca2, BAPTA-AM. Cell invasion was monitored by immuno¯uorescence staining parasites associated with the cell monolayer (Fig. 3A) and by secretion of MIC2 detected by Western blot (Fig. 3B); both sets of data were quanti®ed and are shown graphicallyin Fig. 3C. Treatment with BAPTA-AM, which blocks microneme release (Carruthers and Sibley, 1999), dramatically inhibited parasite invasion of HFF cells, while DMSO (solvent used to disperse BAPTA-AM) or 1/2 BAPTA-AM (a non-chelating derivative of BAPTA-AM) had no effect. In contrast, no change in GRA1 secretion (Fig. 3B) was observed. Cotreatment with a Ca2 ionophore (A23187) in the presence of extracellular Ca2 restored microneme secretion, but only partially restored invasion by BAPTA-AM-treated parasites, possibly as a result of the previously described inhibitory effect of ionophore treatment on invasion (Mondragon and Frixione, 1996). Again, no change in the