Nanoparticles of beta-CDC6 were prepared according to the nanoprecipitation technique. Blank and tamoxifen citrate loaded nanoparticles were characterized by particle size distribution, drug loading, in vitro drug release and transcription efficiency in MELN cells. Particle sizes were between 280-330 nm for blank and drug loaded nanoparticles. Polydispersity indices were below 0.1 for all formulations. Entrapped drug quantities were found to be around 155-160 mu g/ml for both nanospheres and nanocapsules. Nanocapsules and nanospheres showed a similar release pattern, whereas, tamoxifen citrate loaded nanosphere formulation had a significantly faster release profile than tamoxifen citrate loaded nanocapsules. Transcription efficiency of tamoxifen citrate loaded nanoparticles were determined against MELN cells and, in the presence of 17-beta estradiol (E-2), drug loaded nanospheres and nanoparticles did not inhibit the E-2 mediated luciferase gene expression. This study clearly indicates that the antiestrogenic activity of tamoxifen citrate is definitely dependent on the concentration of the E-2.