Recent information from several laboratories suggest that power frequency fields may stimulate cell differentiation in a number of model systems. In this way, they may be similar to pulsed electromagnetic fields, which have been used therapeutically. However, the effects of power frequency fields on phenotypic or genotypic expression have not been explained. This study describes the ability of power frequency fields to accelerate cell differentiation in vivo and describes dose relationships in terms of both amplitude and exposure duration. No change in proliferation or cell content were observed. A clear dose relationship, in terms of both amplitude and duration of exposure, was determined with the maximal biological response occurring at 0.1 mT and 7±9 h/day. Because this study was designed to explore biological activity at environmental exposure levels, this exposure range does not necessarily define optimal dosing conditions from the therapeutic point of view. This study reports the stimulation by power frequency fields of transforming growth factor-b, an important signalling cytokine known to regulate cell differentiation. The hypothesis is raised that the stimulation of regulatory cytokines by electromagnetic fields may be an intermediary mechanism by which these fields have their biological activity.