A number of physico‐chemical and geophysical studies are reviewed suggesting the existence of an unknown physical factor in our environment of a possibly extraterrestrial nature. In order to study, more in detail, the nature of these physical forces, the effect of a metal screen on the sedimentation rate of human blood was analyzed at Leiden, The Netherlands (period 18/11/74–12/6/75) and the influence of different locations of test tubes on the B.S.R. levels at ground level, top floor and cellar of a building at the University Medical Centre at Leiden. The Netherlands (under the same temperature conditions). The result of this pilot study with 465 blood donors showed differences between tests carried out under and outside a copper screen and at different locations in a building.