Mammalian cell gene mutation assays have been used for many years and the diversity of the available systems attests to the varied methods found to grow mammalian cells and detect mutations. As part of the International Workshop on Standardization of Genotoxicity Test Procedures, in Melbourne, Australia, 27-28 February, 1993, various international guidelines were examined with respect to protocol issues. Mammalian cell gene mutation assays generally detect cells which are able to survive in the presence of a selective agent only when a newly arising mutation occurs. Five topic areas were discussed: (4) dose setting, (2) test systems of choice, (3) study design, (4) repetition of tests and (5) criteria of final results. In general, the group felt that the protocols for the conduct of specific gene mutation assays in mammalian cells have been worked out independently and that many aspects of protocol design are not highly contested. Consensus was reached in many areas with respect to elements of protocol design for assays for mutation induction using mammalian cells in culture. A number of issues of a generic nature, common to all in vitro assays was discussed by the larger group.