The lethal dosage of an extract of a toxic strain of Microcystis viridis or purified toxin, microcystin RR was determined against goldfish. The LD₅₀ values of microcystin RR obtained by intraperitoneal injection into goldfish were just above 2 mg·kg^-1 which was about 26 times higher than the known values for mice.
Death of goldfish occurred rather slowly until 5th day, when the observation was terminated following injection of extract. The liver tissue was most severely affected by the intraperitoneal injection of toxin, and hemorrhage, swelling and finally blood pooling in the tissue were observed.
Oral administration of Microcystis viridis as pellets mixed with the algae for 28 d did not cause any mortality but a few fish showed a slight liver damage.