Swiss male mice were exposed chronically to the 50-Hz 5-μT magnetic field present in a laboratory room located above the hospital bus bars and transformer station. During the 90-day exposure, hematological parameters were measured. On day 20 of exposure, a statistically significant decrease was observed among leukocytes, lymphocytes, monocytes, and eosinophils. Differences were no longer observed on day 43. However, on day 90, counts of leukocytes, neutrophils, and eosinophils were significantly lower in the exposed group than in the control group. Differences observed between exposed and control groups on day 20 and day 90 indicate possible effects of exposure on the hematopoietic system.