Lichen samples from the reindeer breeding areas of Sweden have been analysed. Large groups of Lapps in northern Sweden have been measured once a year since 1961. The grazing habits of the reindeer and the fallout pattern imply that there is a delay of 7–10 months in the transfer of 137Cs from lichen to man. From September 1962 to September 1964, the 137Cs content in lichen per unit area has increased by a factor varying from 1.5 to 2.7 (average 2.0). The corresponding 137Cs level in male Lapps has increased by a factor 1.5 to 2.0 (average 1.8) from April 1963 to March 1965.