Tumors arising adjacent to bone have been noted in several species treat! with 90Sr. In mice, squamous cel] carcinomas were reported to originate in the hard palate and sebaceous gland of the external auditory meatus (Nilsson, 1972). Squamous cell carcinomas were also reported in the maxillary sinus in rats (Moskalev et al., 1972) and in the lining of fnternal and external ear in rabbits (Vaughan and Williamson, 1969). Squamous cel] carcinomas of the gingiva have been found to be fmportant Jater-occurring effects in beagles exposed to 905, (by ingestion) from prior to birth untfl 18 months of age and observed for life (Parks et a}., 1980). For the dogs, the gingiva! tumors probably reflect the results of continual frradiation from 90sr in teeth that is maintained at a high level, since teeth show a slower turnover of 90Sr than does the rest of the skeleton (Della Rosa et al., 1964). Fortunately, too few humans have been exposed to radiostrontium at high enough levels for the risk of neoplasia to be estimated. Based dn radium dial painter risks, however, Mays and Lloyd (1972) estimated risks from 99Sr. Their best estimates for the 50 yr risk to an individual for bone sarcoma fron one rad were (1 x3) x 30-8, using a linear model, and (4 + 4) x 10-10, using a dose-squared model. The current ICRP (1977) estimate of risk for 90Sr-tike radiation ts 5 x 10-6 per ren. These numbers can be canpared with a natural incidence of about 5 x 1074,