In recent period we have experienced a further rapid and explosive development of scientific knowledge concerning bacterial resistance to antibiotics. The International Con­ gress of Chemotherapy in London in 1975 dealed with bacterial resistance in many ses­ sions and lectures. We are aware of the fact that also the World Health Organisation in Brussels in 1975 discussed the problem as an integral part of long-term programme in environmental pollution control. A special significance should be ascribed to the discussions concerning the safety of DNA research which ran in several countries as well as on meetings of scientifics involved in the research of recombinant DNA molecules. This problem was also discussed in the meeting of Advisory Committee of World Health Organisation in 1975. On the programme of this Symposium in Smolenice, there are environmental conse­ quences of plasmids, medical aspects of their pools and circulation and their genetics and molecular biology.