Abstract The isospin-symmetry breaking contribution to the proton-neutron mass difference is calculated in a chirally symmetric version of the colour-dielectric model. The basic mechanism is a small difference in the effective constituent up- and down-quark masses which originates in an explicit chiral symmetry breaking term and whose magnitude is determined by the charged-uncharged kaon mass splitting. The result for the model in which the quarks couple to the inverse of the glueball field is M n− M p=2.4±0.5 MeV, where the error reflects only the input data (the experimental value is 1.29 MeV). The baryon mass spectrum is also estimated using the same methods. The results are satisfactory for the case of inverse quark-glueball coupling. They are much less satisfactory for the case of inverse-square coupling. This provides the first example of a prediction which is able to distinguish between these two cases.