We have studied the effects of 50-Hz 100-μT rms magnetic fields on intracellular Ca 2+ concentration in the Jurkat T lymphocyte variant E6.1 using fluorescent probes Indo-1 and Fura-2. We found, however, that the pattern of intracellular Ca 2+ fluctuations also depended on the agent used for cell attachment, in our case the polypeptide poly- l-lysine. In order to isolate possible effects of magnetic field exposure from those of poly- l-lysine, the action of polypeptide on cytosolic Ca 2+ was studied as well. It was found that a 10 −7% concentration of polypeptide triggered prolonged Ca 2+ spiking. Higher (10 −4%) concentrations induced rapid increases in intracellular Ca 2+ followed by high, unstable Ca 2+ levels. The response of these cells to the monoclonal antibody anti-CD3 was also inhomogeneous, similar to one caused by poly- l-lysine. The effect of magnetic field exposure was studied on cells initially exhibiting (1) non-oscillating, low Ca 2+ concentration and (2) prolonged Ca 2+ concentration oscillations. In case (1) the result was negative. In case (2), statistically significant changes were found: the oscillation amplitude was reduced on average by 30%, and the frequency composition was shifted towards higher frequencies.