Beams of diagnostic ultrasound passing through pleural effusions produce an irregular band of complex echoes, when such beams strike aerated lung. To simulate pleural effusions and study this fluid-gas interface, we scanned latex bags of water and air which lay in a water bath beneath a uniform portion of veal rib cage. The general shape of an air-containing object could be determined under these conditions. The display of the upper surface of the gas-filled object was broad and heterogeneous but this zone of distortion was thin in relation to the overlying fluid layer. Fluid thicknesses exceeding 1 cm could be detected under these conditions even when they abut gas-filled structures. These preliminary data suggest that complex artefacts occurring at fluid-gas interfaces during echography of laboratory models simulating pleural effusions would not preclude useful volumentric estimations of overlying fluid layers.