Bacterial systems for the screening of chemical substances for mutagenic potential are receiving widespread attention. Now that many laboratories have introduced these test systems and a large number of substances has been tested, discrepancies in the results have become apparent. To discuss this problem, a workshop was organized at Neuherberg (Munchen) from March 6 to 9, 1977. The workshop was attended by 35 participants who had experience with the test systems, representing various European research institutes and industrial laboratories. The main points under discussion were the following. Comparison of data, with the help of data collecting forms that had been sent previously to the participants. Evaluation of the test systems that are at present in use, with emphasis on the metabolic activation system. Drafting of recommendations for the performance of the tests. Collection of new developments and future needs in this field. Chemicals/CAS: 2 aminoanthracene, 613-13-8; 2 nitrofluorene, 607-57-8; aflatoxin, 1402-68-2; aroclor 1254, 11097-69-1; benzo[a]pyrene, 50-32-8; clophen a50, 8068-44-8; cyclophosphamide, 50-18-0; diethylnitrosamine, 55-18-5; dimethylnitrosamine, 62-75-9; mesylic acid ethyl ester, 62-50-0; methylnitronitrosoguanidine, 70-25-7; mitomycin C, 50-07-7, 74349-48-7; n (2 fluorenyl)acetamide, 27215-65-2, 53-96-3; nitrosomorpholine, 59-89-2; vinylidene chloride, 25323-30-2, 75-35-4