Ionizing radiation is well known as a powerful modifier of plant growth, but the biological effectiveness of the magnetic field is less recognized. Experiments utilizing both agents, alone and in combination, were carried out for the purpose of comparing the effects of and ascertaining the interactions between acute 50,000 R X-irradiation of dormant barley grains and continuous application of a 3000 G steady homogeneous magnetic field during subsequent germination and early seedling development. X-irradiation by itself produced the familiar and significant retardation of germination and seedling growth during 5 days post hydration. The growth effects were of greater magnitude but in the same order of severity as those previously found with lower radiation levels: root length more affected than shoot height or coleoptile height; root number and rate of emergence least affected. Seedling size variability decreased; that for root number increased. Germination in the homogeneous magnetic field caused, by itself, only small, consistent, but statistically insignificant growth inhibition, accompanied by significant increases in variability. Effects on root number, emergence, and intra-embryonic size correlations, however, equalled or exceeded those produced by the radiation. When applied as a post radiation treatment, the magnetic field reduced the radiation effects on growth and root number. As with magnetic effects per se, the degree to which the magnetic field attenuated the radiation damage exhibited in any particular criterion was inversely correlated with the level of radiosensitivity possessed by that criterion. In addition, radiation-induced reductions in variability were lessened by the presence of the magnetic field, and alterations in intra-embryonic correlations, reversed. Synergistic interactions occurred between radiation and magnetic field with regard to the rate of emergence. The severity of growth inhibition produced by ionizing radiation has often been shown to be correlated with effects upon cell production and to reflect the amount of radiationinduced genetic damage, especially chromosomal aberrations. The homogeneous magnetic field, on the other hand, produced its largest effects and greatest attenuation of radiation damage (or the opposite, synergistic interactions) upon those aspects of seedling development which are least radiosensitive and most dependent upon cell enlargement for expression. This suggests that the biomagnetic effects observed must be mediated through physiological processes other than those of primary concern in producing radiation damage.