Nanoparticles (NP) from mixtures of two poly(D,L-lactide-co-caprolactone) (PLC) copolymers, PLC 40/60 and PLC 86/14, with poly(D, L-lactide) (PDLLA) and PCL were prepared:PLC 40/60-PCL (25:75), PLC 86/14-PCL (75:25) and PLC 86/14-PLA (75:25). Tamoxifen was loaded with encapsulation efficiency between 65% and 75% (29.9-36.3 mu g TMX/mg NP). All selected systems showed spherical shape and nano-scale size. TMX-loaded NPs were in the range of 293-352 nm. TMX release from NP took place with different profiles depending on polymeric composition of the particles. After 60 days, 59.81% and 82.65% of the loaded drug was released. The cytotoxicity of unloaded NP in MCF7 and HeLa cells was very low. Cell uptake of NP took place in both cell types by unspecific internalization in a time dependent process. The administration of 6 and 10 mm TMX by TMX-loaded NP was effective on both cellular types, mainly in MCF7 cells.