The cytoadherence of four Plasmodium falciparum malaria isolates (FCR-3, RSA-14, 
15 and 17) to monocytes was used as a measure of the expression of monocyte 
receptors after the monocytes had been exposed to seven antimalarial drugs. 
Quinine, chloroquine, primaquine, pyrimethamine, artemesinin, mefloquine and 
proguanil all down-regulated the expression of monocyte receptors by 40% or 
greater at the therapeutic concentrations of each drug. Each malaria isolate had 
a unique adherence profile for drug induced changes in monocytes. Each drug 
appeared to alter the expression of more than one monocyte receptor. The most 
effective drugs were quinine, pyrimethamine and palludrin and the least 
effective were artemether and mefloquine. The results suggest a previously 
undetected immunomodulatory action of antimalarial drugs.