AbstractSix independent experiments of common design were performed in laboratories in Canada, Spain, Sweden, and the United States of America. Fertilized eggs of domestic chickens were incubated as controls or in a pulsed magnetic field (PMF); embryos were then examined for developmental anomalies. Identical equipment in each laboratory consisted of two incubators, each containing a Helmholtz coil and electronic devices to develop, control, and monitor the pulsed field and to monitor temperature, relative humidity, and vibrations. A unipolar, pulsed, magnetic field (500‐μs pulse duration, 100 pulses per s, 1‐μT peak density, and 2‐μs rise and fall time) was applied to experimental eggs during 48 h of incubation. In each laboratory, ten eggs were simultaneously sham exposed in a control incubator (pulse generator not activated) while the PMF was applied to ten eggs in the other incubator. The procedure was repeated ten times in each laboratory, and incubators were alternately used as a control device or as an active source of the PMF. After a 48‐h exposure, the eggs were evaluated for fertility. All embryos were then assayed in the blind for development, morphology, and stage of maturity. In five of six laboratories, more exposed embryos exhibited structural anomalies than did controls, although putatively significant differences were observed in only two laboratories (two‐tailedPs of .03 and<.001), and the significance of the difference in a third laboratory was only marginal (two‐tailedP= .08). When the data from all six laboratories are pooled, the difference in incidence of abnormalities in PMF‐exposed embryos (∼25 percent) and that of controls ( ∼ 19 percent) although small, is highly significant, as is the interaction between incidence of abnormalities and laboratory site (bothPs<.001). The factor or factors responsible for the marked variability of inter‐laboratory d
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