We report the synthesis of a series of novel phenothiazine compounds that inhibit the growth of both chloroquine-sensitive and chloroquine-resistant strains of Plasmodium falciparum. We found that the antimalarial activity of these phenothiazines increased with an increase in the number of basic groups in the alkylamino side chain, which may reflect increased uptake into the parasite food vacuole or differences in the toxicities of individual FP-drug complexes. We have examined the ability of the parent phenothiazine, chlorpromazine, and some novel phenothiazines to inhibit the formation of beta-haematin. The degree of antimalarial potency was loosely correlated with the efficacy of inhibition of beta-haematin formation, suggesting that these phenothiazines exert their antimalarial activities in a manner similar to that of chloroquine, i.e. by antagonizing the sequestration of toxic haem (ferriprotoporphyrin IX) moieties within the malaria parasite. Chlorpromazine is an effective modulator of chloroquine resistance; however, the more potent phenothiazine derivatives were more active against chloroquine-sensitive parasites than against chloroquine-resistant parasites and showed little synergy of action when used in combination with chloroquine. These studies point to structural features that may determine the antimalarial activity and resistance modulating potential of weakly basic amphipaths.