Cellular uptake of a protein microsphere was investigated in vitro. Fibroin microspheres with an average diameter of approximately 1 mm were prepared with Fluorescein isothiocyanate-dextran for the fluorescent core. Tomography, performed using confocal laser scanning microscopy, confirmed microsphere uptake into the cytoplasmic area of 3T3 cells. Flow cytometry showed that cellular uptake was proportional to the co-incubation time and the microsphere concentration. It also revealed that fibroin microspheres were internalised by 3T3 cells at different rates depending on the cell cycle stage. The following cell stages had higher concentrations of internalised microspheres: G(2)/M > S > G(0)/G(1), when using 0.1mg of microspheres per 10(6) cells. The internalised microspheres per cell also increased in the same order of cell cycle stages when co-incubating cells with 1mg of microspheres. These results provide information that can be used to develop fibroin microspheres for intracellular delivery of large cargos.