This paper describes the results of testing the susceptibility of 60 isolates of the human malaria parasite Plasmodium falciparum from Thailand, and single isolates from five other countries, to five drugs: chloroquine, pyrimethamine, quinine, mefloquine and amodiaquine. The Thai isolates were obtained from patients in three different regions of the country (Chantaburi, Songkhla and Mae Sod), and were first grown in culture by the Trager-Jensen candle-jar technique. Samples were then exposed to a range of concentrations of the five drugs, in Falcon microtest culture wells, for 72 hours, with daily changes of medium (with or without added drug solutions). Presence or absence of parasites was then determined by microscope observations on thin-film Giemsa-stained preparations. Most Thai isolates showed a minimum inhibitory concentration (MIC) for chloroquine of 10(-6) M or higher, and were classified as highly resistant, though one cloned isolate was as sensitive to this drug as a chloroquine-sensitive isolate from West Africa. Similarly most Thai isolates showed a very high resistance to pyrimethamine (MIC 10(-4) M to 10(-6) M), but a few clones were sensitive (MIC 10(-9)) to it. Susceptibility to quinine showed some variation (MIC varied between 10(-6) M and 10(-8) M), and some isolates were thought to be incapable of responding to a therapeutically permissible dose of this drug. Little variation was found in the reaction of any of the isolates to mefloquine or amodiaquine, and by the in vitro technique used in this study, it was found that chloroquine-resistant and chloroquine-sensitive isolates were equally susceptible to amodiaquine. In general the survey showed the existence of a marked correlation between development of drug resistance of Plasmodium falciparum and the extent to which a given drug had been used in Thailand.