Wistar rats were kept for 2 and 4 weeks in conditions of near null magnetic field. The total serum proteins, lipids and glucides, as well as the leucocyte and neutrophil counts showed no significant changes against controls kept in natural magnetic field conditions. A significant monocytosis, a decrease in the circulating phagocyte count and in the phagocytic activity was noticed in null magnetic field. These results suggest that the natural magnetic field significantly influences health both on the ground and possibly in cosmic conditions.