chemical mutagens capable of inducing high frequencies of mutation at high 
levels of survival (17). Concern also stemmed from the realization that man was 
greatly expanding the number of compounds theoretically capable of 
increasing mutation frequencies beyond present ‘spontaneous levels. As 
recently stated at an open meeting sponsored by the United States Department 
of Health, Education and Welfare on the value of selected test systems to 
detect and assess the mutagenic activity of chemicals (21), a human disease 
burden exists which is of genetic origin. Increases in mutation frequency can be 
expected to enlarge this burden, and many classes of chemicals already in the 
environment are known to include genetic toxicants. Although uncertainty 
remains regarding the precise impact such compounds might have upon human 
health, there is justification for apprehension (15). 
Genetic toxicology, a new branch of toxicology concerned with the 
identification and evaluation of DNA-damaging agents (carcinogens, mutagens 
and some teratogens), may be broadly divided into (a) screening tests for 
identification of potential toxicants, (b) procedures for estimating risk, and (c) 
techniques for population monitoring. Screening involves primarily the use of 
rapid, inexpensive assays which detect agents capable of damaging or altering 
DNA. Because DNA is chemically and structurally similar in most organisms, 
and is considered the probable target of genetic toxicants, any organism or 
appropriate part thereof may be theoretically employed as a screening tool. 
Accordingly, viruses (18), a variety of microbial systems (3,36), cultured 
animal cells (14, 39, 26), Drosophila (2, 48), and various subcellular assays 
designed to measure effects directly on DNA (47, 49) are widely used for 
screening purposes. Several short-term tests utilizing intact mammals are also 
available for screening (31). 
This paper is concerned with the application of an in vitro mammalian cell 
assay utilizing nutritional markers as an indicator system for genetic toxicants 
detected as mutagens. Major objectives are to (a) outline techniques for 
measuring the acute toxicity of chemicals to cultured cells, (b) qualitatively 
describe the CHO Cell/BrdU-VL system as an assay for mutation, (c) present 
data relative to the mutagenic potential of a series of compounds known to 
accumulate in the tissues of edible marine organisms (41,45, 52) or which have 
been associated with the occurance of neoplasias in such organisms (8, 51), and 
(d) illustrate some additional end points, as well as some potential problems, 
pertinent to the application of in vitro cell assays. A detailed description of 
equipment, reagents, special techniques and experimental procedures relevant 
to the CHO Cell/BrdU-VL system will not be given here. A general protocol for 
this assay has been published by Kao and Puck (28). Our modifications to their 
procedure will be described elsewhere (33). 
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