Abstract: In connection with the inoculation of rice fields with nitrogen-fixing blue-green algae as biological fertilizer,the question has arisen as to possible contra-indications for the application of fungicides and insecticides,which are a necessity for the control of enemies and diseases of the rice plants.Experiments have therefore been made to test the effect on the blue-green algae of some commonly used chemicals and their combinations. Lime,copper sulphate,a mixture of lime and copper sulphate,Bordeau's mixture,“666”, ceresan,and E605,have all been tested against the cultures HB 686(Anabaena azotica),and its growth behaviour studied under controlled condition.The results of our experiments show that within the dosage range in which these chemicals are applied in the field,there are no conspicuously harmful effects on this alga.Moreover,it is of considerable interest that small amounts of these chemicals often have a growth promoting effect on the Anabaena azotica.