The metabolism of environmental 137Cs in twenty adult males was studied for a period of 2 years. A material balance was made of the ingestion, excretion and assimilation of the radionuclide by the body. The fate of the radionuclide was observed, and simple relationships were established to assess and evaluate the hazard of 137Cs to man. An apparent correlation was observed between the 137Cs content in man and urine. Ingestion via milk and dairy products accounted for 86 per cent of the total 137Cs assimilated. The biological half life for adults averaged 93 days, ranging from 68 days in the summer to 130 days in the winter. The 137Cs content was much higher in children's urine than in adults' urine; it appears that the biological half life for children is much less than that for adults.