KL-KS mass difference is computed in a left-right symmetric model with three generations of quarks. Under the assumption of "manifest" left-right symmetry the mass of the right-handed charged gauge boson consistent with KL-KS constraint appears to be rather high. A low mass right-handed gauge boson can be accommodated in models with broken "manifest" left-right symmetry. The latter scenario has many non-trivial predictions for the decay of heavy (c, t, b) quarks. © 1983 Springer-Veralg.