Summary The phenothiazinium salt methylene blue [3,7-bis(dimethylamino)phenothiazinium chloride] is the oldest known synthetic antimalarial drug, its clinical efficacy having been reported in 1891. The role of methylene blue in the evolution of the modern antimalarial armoury is often unappreciated, yet it can be linked directly to standard drugs such as chloroquine and its congeners. Also, in the face of increasing plasmodial resistance to modern antimalarials, phenothiazinium derivatives have again featured as lead compounds in drug research. The precise mode of action of methylene blue and its commercial analogues against Plasmodium spp. remains a cause for conjecture, having been variously described as nucleic acid intercalation, food vacuole basification, parasite redox cycle interference and haem polymerization inhibition. That the activity of the series may be due to more than one route – i.e. a multifactorial activity – underlines the utility of these compounds in antimalarial research either as single drugs or as adjuvants (partners in a drug combination), particularly in the face of resistant parasitic strains.