Magnetic field strengths in high-redshift galaxies - Can the galactic dynamo be tested?

The hypothesis that the population of strong H I absorption systems is responsible for a significant excess Faraday rotation measure in quasars is discussed, and it is concluded that the case is unproved, in contrast to a recent analysis by Wolf et al. (1991). The limitations and pitfalls inherent in attempts to derive firm magnetic field strengths from the existing integrated rotation measures of quasars are discussed, and it is shown that although it is premature to use integrated quasar rotation measures to either confirm or rule out particular mechanisms of magnetic field amplification in galaxy disks, the present observations may call for reexamination of current theories of large-scale magnetic field generation. The sources of rotation measure in the double quasar 0957+561 are also discussed.