In this paper we describe a method for estimating the internal parameters of the left and right cameras associated with a stereo image pair. The stereo pair has known epipolar geometry and therefore 3-D projective reconstruction of pairs of matched image points is available. The stereo pair is allowed to move and hence there is a collineation relating the two projective reconstructions computed before and after the motion. We show that this collineation has similar but different parameterizations for general and ground-plane rigid motions and we make explicit the relationship between the internal camera parameters and such a collineation. We devise a practical method for recovering four camera parameters from a single general motion or three camera parameters from a single ground-plane motion. Numerous experiments with simulated, calibrated and natural data validate the calibration method.