A marked correlation between the abundance of M. aeruginosa blooms and high temperature, pH, N and P content of the pond water was observed during studies. Extract of M. aeruginosa showed toxicity to mice and showed the absorption maximum at 236 nm. Co-chromatography and HPLC analysis using standard microcystin showed that the compound present in MT-2 is identical to standard microcystin-LR. Intraperitoneal administration of crude and purified toxins of M. aeruginosa caused death of mice after 40-45 min. Microcystin content in water was also determined and revealed the presence of microcystin-LR in both the ponds Laxmikund and Durgakund respectively. The impact of partially purified microcystin was studied on plants system. Germination percent, growth and antioxidative activity of four plants (rice, cauliflower, mustard and onion). A drastic inhibition in percent germination was observed in mustard, cauliflower and onion seeds was observed in the presence of 2.5µg/ml microcystin. However, there was no significant effect of this concentration of MC on seed germination in rice. Growth (evaluated in terms of shoot and root length, fresh weight and total chlorophyll content) of all four plant seedlings was also significantly affected at the same concentration of microcystin. During antioxidative studies, significant increase in activity of SOD, POD and CAT was observed in the presence of different concentrations of microcystin in seedlings of all plants tested.