Even in the absence of antiprotons in the primary cosmic rays, a flux of secondary antiprotons will be produced in collisions between cosmic rays and interstellar gas. The predicted antiproton fraction increases with increasing cosmic-ray confinement, so that observations of antiprotons will provide a probe of models of cosmic-ray confinement. It is shown that the expected antiproton fraction (for E (p-bar) > or approx. =10 GeV) ranges between 2.3 x 10/sup -4/ for the ''leaky box'' model and 18 x 10/sup -4/ for the ''closed box'' model. In addition, attention is called to the fact that a detection of cosmic-ray antiprotons at, or above, a level of 2 x 10/sup -4/ will provide a valuable lower limit to the antiproton lifetime (and hence a check on the validity of CPT invariance).