The influence of the external magnetic or electric field on the ordering of liquid crystals in uniaxial nematic phase is considered. Thermodynamic and fluctuation mechanisms of field action on the tensor order parameter are taken into account. The thermodynamic input is proportional to the square of the field. The fluctuation input is linearly dependent on the field, and is caused by suppression of the director fluctuations in the external field. It is shown that both mechanisms in liquid crystals with negative anisotropy of permittivity or magnetic susceptibility produce optical biaxiality under the action of the field. This effect can be considered as a biaxial analogue of the Kerr or Cotton-Muton effects in uniaxial systems. The possibility of an experimental observation of these effects is discussed. Copyright (C) 1999 Elsevier Science B.V.