Sera from different malaria endemic regions of the Republic of the Philippines were compared for their ability to inhibit growth of Plasmodium falciparum in vitro. Dialyzed serum was added to synchronous cultures containing schizonts for either the total 48 hr test period or only the last 24 hr in order to analyze the effects on erythrocytic invasion and intraerythrocytic growth, respectively. Reduction in 3H-hypoxanthine uptake was used to determine the percent of inhibition compared to nonimmune serum. One hundred seventy sera from Mindanao and Palawan in the South, the centrally located island of Mindoro, and Luzon in the North, were tested against 4 P. falciparum strains from the Philippines and 1 from Africa. Indirect fluorescent antibody titers were not predictive of inhibition. Inhibition of merozoite invasion rather than intraerythrocytic parasite growth is suggested by this study. Generally, sera were more inhibitory to parasite strains from the same geographical area than to those from more remote areas.