Magneto-acoustic instability driven by cosmic rays and the waves that they excite was first studied by McKenzie and Webb and by Zank. In the present paper this instability is reconsidered. In general, the cosmic-ray plasma system is unstable. However, depending on how the time scale is chosen, the results and interpretation may be different from those of McKenzie and Webb and of Zank, particularly in the long- and short-wavelength limits. To illustrate this instability in an astrophysical context, we take our Galaxy and supernova remnants as examples. In those examples, length scales shorter than 2.5 kpc and 1.5 pc respectively are susceptible to this instability, and the growth times are of the order of 4 x 108 and 1.5 x 104 yr respectively.