This invention relates to the use of certain C-10 substituted derivatives of artemisinin of general formula (I) in the treatment and/or prophylaxis of diseases caused by infection with a parasite, certain novel C-10 substituted deriviatives of artemisinin, processes for their preparation and pharmaceutical compositions containing such C-10 substituted derivatives. The compounds are particularly effective in the treatment of malaria, neosporosis and coccidiosis. ##STR1##