Mesoporous silica nanoparticles are being explored as versatile tools for various biomedical and biotechnological applications including disease diagnosis, drug delivery, and intracellular imaging. In this paper, the synthesis and characterization of a fluorescent hybrid mesoporous silica nanomaterial, which is noncytotoxic and shows great potential for "in-cell" bioimaging applications, will be described. The hybrid mesoporous material has been obtained by confining highly fluorescent organic dyes, belonging to the indocyanine family, within the channels of mesoporous MCM-41. To explore the dispersion of the dye inside the mesoporous channels and the formation of dye aggregates, several hybrid samples with increasing dye/MCM-41 loading (up to 100 mg/g) were prepared. A uniform distribution of monomeric 1,1'-diethyl-3,3,3',3'-tetramethylindocarbocyanine iodide has been achieved at low dye loading (1 mg/g), as evidenced by photoluminescence spectra and lifetime, while a progressive formation of J-aggregates is induced by an increase in the dye loading. To elucidate the properties of the dye immobilized in mesoporous MCM-41, a detailed physical chemical characterization by structural (X-ray diffraction), volumetric and optical (Fourier transform infrared, diffuse-reflectance UV-vis and photoluminescence) techniques has been performed. By ultrasonication of the bulk material, nanoparticles of 2-20 nm diameter were obtained. Biocompatibility, endocytic uptake, and intracellular compartmentalization of such fluorescent nanoparticles were investigated in mammalian cultured cells.