Malaria remains one of the leading causes of morbidity and mortality worldwide which is partly due to the emergence of the parasite resistance to antimalarial drugs. In vitro testing of drugs allows an early detection of resistance. The common parameter used for the monitoring of resistance is the concentration that inhibits 50% of the parasite's activity (IC(50)). Various methods of calculation are already used but none of them consider new available tools nor display the precision of IC(50) estimation. We proposed an approach based on the inhibitory sigmoid E(max) model, which is often used in pharmacology, with estimation of IC(50) through nonlinear regression using a standard function of the R software. To facilitate the usage of that tool we have developed an online version available on the website ICEstimator (ICEstimator website http://www.antimalarial-icestimator.net/, 2009) [1]. This website is used by various teams in the world and the user can do the analysis without knowing R using the GUI. This article describes version 2.1 of this website and shows illustration on five different real examples.