This study was conducted to examine the stages showing the antimutagenic effects on the microbial mutation by addition of the juice extracted from small water dropwort. It was not able to find out the signal showing the genic derepression or change of gene repair system by addition of the juice. And it was hardly possible to expect the conversion of 2-AF to inactive form by the juice. however the longer 2-AF and S-9 mix were contacted before addition of the juice, the stronger the microbial mutagenisity of 2-AF was, and after addition of the juice, the mutagenicity was decreased rapidly. It seems that some components in the juice act as inhibitor of a enzyme in S-9 mix, and block the conversion of 2-AF to the ultimate mutagen.