Retention of 6oCo given intravenously or orally as 60CoC1, to human subjects was measured by whole-body counting for periods of up to 1018 days. Intravenous 6oCo was retained for long periods, as much as 9-16% of the dose being eliminated only with a biological half-life of about 2 yr. The absorption of orally administered 6oCoCl, depended on several factors, especially on the amount ofstable cobalt given. Only 5% or less of a trace dose (containing less than lpg Co) was absorbed but this increased to more than 20% when larger quantities of stable cobalt (1.2 mg) were given. The absorbed fraction of an oral dose was apparently retained by the whole body in a similar way to "Co given intravenously. Results of serial counts over the liver using a collimated NaI detector, indicated that 6oCo was concentrated in this organ in excess of the average whole-body concentration even in subjects measured after 1000 days. Our estimates suggested that the amount of 6oCo present in the liver was, on average, about one fifth of the total body content.

The amount of 6OCo absorbed by rats given 6oCoCl, in drinking water was only about one quarter of the amount absorbed by rats given 6oCoCl, by gastric intubation. When allowances were made for this diflerence, retention data obtained in single-administration studies, in which rats were given 6oCoCI, by gastric intubation, closely predicted the organ burdens acquired by rats allowed to drink 6oOCo-water continuously for given periods. The subsequent whole-body retention of 6oCo by rats restored to normal drinking water was also accurately predicted and this result confirmed the value of the half-life of the long-term wholebody retention component observed in single-administration studies. Using the values of organ burdens at equilibrium, derived from the results of this study, calculations were made of the dose rates which would be delivered to organs of man if the metabolism of cobalt were similar in rat and man. On this basis the lower large intestine (LLI) would be the critical organ for ingested 6OCo and the maximum permissible concentration (MPC) of 6oCo in drinking water, applicable to individual members of the public, would be 4.5 x lop5 pCi/ml. This MPC agrees with the presently accepted ICRP value. The possible differences between actual dose rates to organs of man and those predicted by extrapolation from rat data are discussed.