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Notes: Abstract The introduction of foreign cells (e.g. ascites tumor cells) into laboratory mammals and their subsequent recovery after treatment of the host with exogenous chemicals to determine the induction of genetic effects (e.g. chromosomal aberrations) is a technique which has been employed for more than 20 years. The use of bacteria as indicators of induced point mutations was first described by Legator et al. (1969). In their technique which they called a host-mediated assay, the microbes (e.g. Salmonella typhimurium) were injected into the peritoneal cavity of mice and, thus, exposed to potentially mutagenic metabolites of the compound under test. Today a wide variety of genetical changes can be detected in several indicator organisms. In addition to the histidine-requiring strains of Salmonella which allow the detection of different types of back-mutations, some other enterobacteria have proven useful. Auxotrophic strains of Serratia marcescens to detect back mutations, and Escherichia coli bacteria in which both forward and back mutations can be assayed simultaneously in several different genes. With fungi or fungal spores as indicators further effects of genetical importance can be determined, e.g. the consequences of recombination processes such as mitotic gene conversion and mitotic recombination in the yeasts, and of deletions in Neurospora crassa (conidia). After the successful development of methods to measure the induction of point mutations in cultured mammalian cells, it is also possible now to use established animal cell lines (e.g. mouse lymphoma cells) in a host-mediated assay provided isogenic or compatible hosts are available. During the past several years the route of administration of the indicator cells and the distribution of these cells within the animal body have been studied. Efforts have been made to bring the indicators into close contact with the reproductive organs or within the liver because this is the organ where most foreign compound metabolism is known to occur. Depending on the inoculation technique (in situ, intraperitoneal, intravenous) it is now possible to recover indicators out of testes, liver spleen, lungs and peritoneal cavity of treated animals in quantities large enough to perform genetic tests. A further improvement is the introduction of indicators within the intestinal tract of rodents; the first experiments along this line using Salmonella seem promising and have opened the way toward using common representatives of the intestinal flora such as Escherichia coli. In general it appears that the host-mediated assay technique is a useful tool to use in assessing the degree of mutagenicity and the organospecificity of foreign chemicals in living mammals and that it should be used until techniques that allow the detection of genetic events in the cells of the different organs have been developed. In the near future, host-mediated assays will have to be used to assess more quantitatively the mutagenicity of the numerous chemicals that have been found genetically active by using direct microsomal assays. Further improvements of host-mediated assays are necessary. For instance, there is as yet no practical method for retention of microbial indicators long enough within the blood stream of living animals to perform genetic tests. This would be desirable for pharmaceutical reasons, e.g. for determination of the kinetics of appearance and removal of mutagenic factors within the blood stream. Preliminary experiments indicate that bacteriophages might be useful indicators, since they are present in blood in quantities sufficient for genetic analysis more than 24 h after intravenous injection. In contrast to all other indicators used so far, bacteriophages do not show any metabolism outside their bacterial hosts that might interfere with the mammalian metabolism. Studies are presently under way to determine what types of mutations or of genetic alterations are best detected after extracellular phage treatment.