We investigated placental malaria infection and blood chloroquine concentration in pregnant women to assess a new method of monitoring the chloroquine resistance (CQ-R) of Plasmodium falciparum. We studied two groups of women: 96 women taking the usual prophylactic treatment of 300 mg chloroquine per week and 108 controls given no treatment. We carried out placental apposition, a thick smear and determination of blood chloroquine concentration after delivery. Malarial infection of the placenta was more frequent and the density of parasites higher in controls than in women given prophylaxis. Blood chloroquine concentrations were adequate in all women given prophylaxis (above 200 ng/ml, with a mean of 500 ng/ml). Twenty-six per cent of placentas in the treated group and 51% of placentas in the control group were infected. The placental infection rates for the two groups were compared. The frequency of chloroquine-resistant strains was about 50% (2 or 3 times higher than the CQ-R frequencies obtained in studies using in vivo tests). The simplicity of placental apposition makes this technique potentially valuable. However, determination of blood chloroquine concentration is also required and can only be carried out in a reference center.