It depends largely on your extract, i.e. its solubility in medium used for MTT assay. Try to use DMSO as medium since it can dissolve a broad spectrum of extracts. However, the concentration of 100 mg/ml is high, use it only if there is no activity on lower concentrations.
Thanks for the responses. Due to extract limitations, decided to start at a concentration of 20mg/ml with subsequent 2-fold dilutions and found that at 20mg/ml, it is toxic to the cells.