Primary cultures of Macaca mulatta hepatocytes infected with sporozoites of Plasmodium knowlesi, P. cynomolgi (Cambodian strain), and P. cynomolgi bastianellii were exposed in vitro to 7 antimalarial compounds. The number of exoerythrocytic schizonts present after 4–7 days of culture was used to assess the activity. With pyrimethamine, proguanil, cycloguanil, primaquine, and 2 of its analogues (WR242511 and WR238605), marked inhibition of schizont formation could be achieved at concentrations below those causing a cytotoxic effect on the host hepatocytes. Chloroquine had only minimal schizonticidal activity at a concentration that produced severe hepatocyte toxicity. This simian in vitro system provides a reliable model for screening antimalarial compounds and for investigating their effects on the hepatic stage of malaria parasites.