Five new pyridazine derivatives (1 salt and 4 pyrrolopyridazine cycloadducts) were prepared and tested in vitro as antimicrobial compounds. Some of them have proved to have a remarkable activity against different micro organisms (germs and fungi). The influence of microwave irradiation concerning cycloaddition reactions of pyridazinium ylides was studied. Stereo- and regiochemistry involved in these reactions are also discussed.