The physicochemical parameters influencing particle crossing through the epithelial barrier have been largely studied and size has been shown as a crucial factor. Most part of this work has been done with polystyrene (PS) particles; however, few studies were conducted with well characterized biodegradable polymeric nanoparticles, more suitable for drug delivery. The aim of the present work was to study the influence of the size of well characterized biodegradable polymeric particles on cellular uptake by Caco-2 cells. Poly (d,l-lactide-co-glycolide acid) (PLGA) particles loaded with a fluorescent dye, 3,3'-dioctadecyloxacarbo-cyanine perchlorate (DiO), were prepared by the emulsion evaporation process. Five batches of particles with narrow size distribution (100, 300, 600, 1000, and 2000nm) were produced using selective centrifugation. Surface properties (zeta potential, hydrophobicity and residual surfactant rate) were similar among all batches. Preliminary study showed that after growth and differentiation, cell counting differed from plate to plate. Therefore, a quantitative method using fluorescence spectroscopy was developed to estimate the number of particles interacting with a single cell. The results were compared to the interaction obtained with polystyrene particles, commonly used as particle model. The interaction was clearly dependant on particle size and concentration. Particles in the range of 100nm presented a higher interaction when compared to larger particles. More than 6000 (d,l-lactide-co-glycolide acid) particles and 200,000 polystyrene particles were quantified per cell. Cellular localisation of particles by confocal microscopy showed the association of the poly (d,l-lactide-co-glycolide acid) particles with the cells. Small particles were observed intracellularly, whereas particles larger than 300nm were associated with the apical membranes. Interestingly, some of the 100nm PLGA particles were localized in the nuclei of the cells.