Effects of a 5, 000 gauss magnetic field on three biological systems were investigated. (1) Chinese hamster ovary (CHO) cells, growing in a plastic flask, were exposed to the magnetic field with full of F10 complete medium for 8 hours at room temperature. After the exposure, the cells were dispersed and replated for the examination of cellular growth and multiplicity, and of colony forming ability. (2) Naturally synchronous plasmodia of Physarum polycepharum were exposed at S-G2 or M-S-G2 for 8 hours at 25°C, and examined if mitosis was delayed. (3) The fertilized or early cleavage eggs of Xenopus laevis were also exposed (up to 72 hours), and their hatchability and delay in the embryonic development were checked. The results of the present study indicated that such short time exposure does not produce any biological disturbance in all of these three systems.