Leos Mandel Leos Mandel, a leading scientist of the Department Immunology and Gnotobiology, Institute of Microbiology in Prague, Czech Academy of Science, passed away suddenly (heart attack) on 3 August 1999. Leos was a world renowned Czech gnotobiologist who, in the 1950's as a member of the immunology group of Jaroslav Sterzl, was one of the first to establish colonies of germ-free animals in Central Europe. Germ-free piglets were used as a unique immunological model to study the interaction between microorganisms and the immunologically inexperienced host. His scientific interest concentrated on foetal haematology, radiobiology of the immune system, effects of non-antigenic diet and bacterial immunomodulators in germ-free animals. In the middle of the 1970�s, he began to draw a series of pictures featuring microbes endowed with human vices and virtues which became well-known (the book containing his selected pictures will appear this autumn). Leos� vitality and humor were a source of inspiration, we will miss him very much. Ilja Trebichavsky and colleagues