In summation I start this session with the opinion that in vitro methods cannot be considered as adequate replacements for entire animals at the level of regulatory testing. But, when used to identify mechanisms of action, they can be extremely useful as secondary stage supporting studies. They are of doubtful value for general purpose, broad spectrum screening of single chemical entities or for priority selection of unrelated chemicals. They can be of value for priority selection of homologous series with a known, specific effect on reproduction or development. Such situations are most likely to be present in chemical and drug manufacturing industries where judicious use of in vitro methods in an integrated approach could reduce the number of failures at the later stage of full scale testing. Whether I will need to revise my opinions at the end of this session will depend upon what our speakers have to offer.