the radioactive material to sensitive cells and the potential duration of contamination are important and are in part depen- dent on the source of the contamination. If biological cycling is considered, the nature of the contamination of each organism in the food chain affects the availability of the radioactive materials to the next higher organism in the chain, 1.e., materials which have been absorbed or metabolized once are more likely to be absorbed in the next step than are sur- face contaminants. In an evaluation of the sources of radioactive contamination, the tissues of an organism may be grouped into the following categories: (1) tissues, such as liver, bone and muscle, which have only those isotopes absorbed from the blood and (2) tissues such as skin, gill, shell and digestive tract, which may have "surface™ contamination from externally adsorbed or adhering materials in addition to absorbed isotopes. (Radio- active materials in the digestive tract are considered surface contaminants as long as they have not been absorbed). The immediate sources of surface contamination are direct and indirect. The direct sources are the fallout particles and the induced radioactive materials in the sea water, air, or substrate, and isotopes of those materials that are soluble in water. Indirect sources are other radioactive organisms which are ingested by the specimen or commensal with it. 4.9.1 Speck Contamination Autoradiographs have shown that the distribution of - 86 -