In biology, chemistry and physics one often deals with systems that can occur in two forms that are each other's mirror image. A new magneto-optical effect called magnetochiral anisotropy (MChA) is described that establishes a link between chirality and magnetic fields and that can discriminate between the two enantiomers (mirror images) of chiral systems. It is shown that through MChA a magnetic field can lead to an enantiomeric excess in a photochemical reaction with unpolarized light. The existence of MChA can be deduced from general symmetry arguments, and the question comes to mind whether this effect can manifest itself in other magnetotransport phenomena in chiral media. Theoretical evidence is presented that indeed MChA is a general phenomenon, and its existence is demonstrated experimentally in the electrical conductivity of carbon nanotubes.