Recent progress in 3-D, immersive display and virtual reality (VR) technologies has made possible many exciting applications, for example interactive visualization of complex scientific data. To fully exploit this potential there is a need for "natural" interfaces that allow the manipulation of such displays without cumbersome attachments. In this paper we describe the use of visual hand gesture analysis and speech recognition for developing a speech/gesture interface for controlling a 3-D display. The interface enhances an existing application, VMD, which is a VR visual computing environment far molecular biologists. The free hand gestures are used for manipulating the 3-D graphical display together with a set of speech commands. We describe the visual gesture analysis and the speech analysis techniques used in developing this interface. The dual modality of speech/gesture is found to greatly aid the interaction capability.