Chitosan-coated oxygen microbubbles of average diameter 2.5 mum, narrow size distribution and spherical shape were prepared. A core-shell structure was evidenced by fluorescence microscopy using fluorescent microbubbles. Such microbubbles can be a therapeutic device for vehiculating oxygen to hypoxic tissues, provided they show proper permeability and diffusivity properties and are non-toxic. Our study proves that oxygen is efficiently delivered both in 'in vitro' and 'in vivo' preparations, and can be conveniently metabolized reversing the cellular hypoxic response. Moreover, toxic effects were investigated in human blood and in cultured cells and no evidence for them was found.