Dielectric permittivity, ε22, spontaneous polarization, Ps, and coercive field, Ec, of triglycine sulphate (TGS) crystals doped with d-l alanine, orthonitroaniline, paranitroaniline or aniline molecules have been measured from room temperature up to the transition point. A remarkable displacement of the hysteresis loops of polarization along the direction of the field axis and a broadened transition from the polar to the non-polar phase are detected. An increase in the value of Ps and a decrease of both ε22 and Ec are also observed. The changes in both ε22 and Ps attributed to the creation by the dopants of an internal electric field, Eb. The internal bias affects the Curie constant and εmax in a way similar to that introduced by an external bias. The value of Eb in the vicinity of the phase transition and the parameters of the statistical dipolar theory of ferroelectricity have been calculated. ©1989 The Physical Society of Japan