To develop criteria for the diagnosis of resistance to chloroquine using an in vivo test, we examined published records of early clinical trials of quinine and chloroquine against Plasmodium vivax. These data established the timing of relapse by tropical P. vivax relative to therapy by these drugs. The first relapse occurred 17 days after initiating and three days after terminating quinine therapy. The median day of relapse was day 23, and 59% of the patients had relapsed by day 30 (n = 333). In contrast, no relapse occurred until day 36 following chloroquine treatment (n = 256). Data from our laboratory may help explain this difference; among 91 Indonesian patients with malaria, the mean whole blood levels of chloroquine (CQ) and desethylchloroquine (DCQ) were 141 ng/ml (95% confidence interval = 115-167) on day 28 after initiating standard therapy (25 mg base/kg in three doses over a 48-hr period). This exceeds the estimated minimal effective concentration of chloroquine (100 ng/ml of whole blood). Thus, chloroquine lingering in the blood for at least 28 days after starting standard therapy was sufficient to eliminate or at least suppress chloroquine-sensitive tropical P. vivax. We conclude that a parasitemia by P. vivax recurring in the 28 days after full compliance to standard chloroquine therapy demonstrates resistance. If the recurrence appears before day 16, it is almost certainly a recrudescence and between days 17 and 28 it may be either a recrudescence or a relapse by chloroquine-resistant parasites. Recurrences beyond day 28 could be relapse by chloroquine-sensitive P. vivax.