Nanoparticles are solid colloidal particles ranging in size from about 10-1000 nm. Depending on the method of preparation, the following potentially toxic impurities can be present in the nanoparticulate suspensions: organic solvents, surfactants, residual monomers, polymerization initiators and large polymer aggregates. The potential use of nanoparticles in an oral or parenteral route implies an intensive purification. In the present work, we investigated the purification by crossflow microfiltration of nanoparticulate suspensions prepared by an emulsion-diffusion technique. The purification process involved two steps, first a concentration step where the preparation is concentrated by a factor of 115, then a diafiltration step, where the feed volume is kept constant by adding continuously pure water. It was shown that polyvinyl alcohol (PVAL) and ethyl acetate concentrations decreased to an acceptable level, after 40 min of concentration and 2 It of diafiltration. The nanoparticulate suspension remained stable after 9 weeks of storage. Indomethacin loaded nanocapsules were purified without loss of drug efficiency. Also, the influence of process parameters as crossflow flowrate on the PVAL amount present in the feed was investigated. (C) 2003 Elsevier B.V. All rights reserved.