Abstract: The influence of some factors including the erythrocyte concentration, the starting rate of parasitemia and the cultivation time on the ire vitro determination of chloroquine sensitivity of Plasmodium falciparum were studied. The results showed that the chloroquine concentration required to produce 50% or 95% reduction of the parasitemia (SC50 or SC95) was positively related to the observed erythrocyte concentrations (1.25-10%), their r values being 0.94 and 0.92, respectively, and also related to the starting rate of parasitemia (0.5-4%), their r values being 0.95 and 0.99, respectively. 48-hour cultivation was considered to be suitable.