We report a rapid (2-3 hr) in vitro test for chloroquine resistance in Plasmodium falciparum. The test is based on the inhibition of chloroquine efflux by verapamil; it is performed by diluting infected blood in culture medium and incubating the diluted blood for 60 min at 37°C with 50 nM 3H-chloroquine, with and without 10 μM verapamil. The test can be performed with the ring stage parasites in the blood of infected patients and in the presence of white cells, platelets and anticoagulants (heparin, EDTA, or citrate). Although the test can be performed in triplicate with 20 μl of blood and specimens may be kept in anticoagulant at 4°C for up to 24 hr, parasitemias < 0.1% limit the sensitivity of the assay. Inhibition of chloroquine efflux by verapamil may permit the rapid identification of chloroquine resistant P. falciparum in blood specimens from infected patients.