Evidence is presented that suggests that a large part of the alpha activity observed in certain samples of grass may originate as a decay product of atmospheric radon which is deposited onto the grass by rainfall. Results are reported from measurements of alpha activity of samples of grass and soil collected in the southern part of Great Britain. Measurements were also made on tissues from lambs from the same region. Polonium-210 was found concentrated in the kidney. Observations of the decay of total alpha activity indicated that polonium-210 occurs in the absence of lead-210. (C.H.)