The responses of macaque medial superior temporal (MST) cells to translation and to the optic-flow components-rotation, expansion/contraction, and deformation-were examined with particular regard to the speed tuning of MST cells for optic-flow stimuli and the effect of removing speed gradients from those stimuli. The use of position invariance as an indispensable criterion for assessing the authenticity of responses to optic flow is reviewed. By extending the scope of testing to include higher speeds it is found, in contrast to in previous reports, that MST cells generally respond to optic-flow components with a speed-response profile which is tuned for a particular range of speeds. Removal of the speed gradient had little effect on this observation. These and other properties of MST cells lead to the conclusion that one of the major functions of MST is the detection and encoding of self-motion.