The strong electric currents used for heating purposes in welding and steel industries set up related magnetic fields (generally 0.1--10 mT and 50 Hz). Such fields 10--50 Hz, 0--40 mT) were used to induce visual light phenomena, magnetophosphenes. Threshold values for magnetophosphenes were determined as a function of magnetic field frequency as well as colour and intensity of the background illumination. A typical sensitivity maximum was found at 20--25 Hz. Differences between volunteers with normal colour vision and colour defective ones were observed. The frog retina was exposed to the same type of fields. Retinal activity, induced by the fields, was registered from the ganglion cell layer by means of microelectrode technique. The results indicate that magnetophosphenes are generated in the retina and in the same channels that are normally propagating signals induced by light of different qualities. The findings may be of guidance when formulating threshold limit values for extremely low frequency electromagnetic fields in industry.