A number of drugs, including CQ, potently inhibit the formation of Hz in vitro. Provocatively, in vitro IC 50 for Hz crystallization sometimes (but not always) correlates with antimalarial IC 50 in vivo [14] . However, this does not mean that every quinoline drug acts on the same chemical form of FPIX or at the same pH optimum. The kinetics of Hz inhibition for different drugs against various CQ sensitive (CQS) or CQR strains of malaria are just beginning to be defined in vivo, because crystals form in an irregular shape and are exceedingly difficult to image quantitatively within the live parasite [15] .