The in vitro microculture technique was used to develop a relatively simple bioassay for estimating chloroquine (CQ) in plasma or serum. Chloroquine concentrations were determined by multiplying the maximum inhibitory dilution of plasma/serum required to inhibit growth of the CQ-sensitive FC27 isolate of Plasmodium falciparum by the minimum inhibitory concentration of CQ against the same isolate. Human serum samples spiked with CQ gave similar measurements using both bioassay and high-performance liquid chromatography. The antimalarial activity of plasma or serum samples collected from 13 patients treated with CQ was equivalent to the sum of the combined activity of CQ and its metabolite, mono-desethylchloroquine. The concentration of these components using the bioassay could be expressed conveniently in terms of CQ concentration equivalents. This bioassay can be used to estimate drug concentrations without the use of sophisticated methods or equipment. Since it is based on the microculture technique, it can be easily carried out in conjunction with the drug susceptibility test to assess CQ treatment failures in malaria patients.