The stiffness of individual hair bundles of hair cells from the frog sacculus was measured using calibrated quartz probes. For displacements of up to 1 micron in either direction (angular deflections up to +/- 0.13 rad) the stiffness was constant. The stiffness did not depend on whether the bundle was in compression or tension. At first approximation, the stiffness was inversely proportional to the square of the height of application of the force above the apical surface of the hair cell. This is consistent with pivoting of the stereocilia within the hair bundle about their points of insertion into the cuticular plate. The pivotal stiffness of the bundle was approximately proportional to the bundle's cross-sectional area and hence to the number of stereocilia of which it is composed. It is inferred that the contribution of the kinocilium to the total bundle stiffness is small. It is concluded that applied forces are shared almost equally amongst all stereocilia, that there is relative shear between neighbouring stereocilia during bundle deflection and that each stereocilium contributes a pivotal stiffness of 0.49 +/- 0.15 X 10(-15) N X m X rad-1. The measured stiffness of the stereocilium is consistent with a structure which bends mainly at the tapering insertion point. The data are also consistent with little cross-linking here between actin filaments. The nature of the links between stereocilia in the hair bundle is also discussed.