Esterases show a great potential in plant biotechnology. In this article we report a simple method for assessment of their activity using fluorescein diacetate. The method was applied to cell suspension cultures of tobacco BY-2 line, a culture of early somatic embryos of Norway spruce on a semisolid medium and to a suspension culture of onion. We demonstrated that the intracellular esterases can be used for construction of growth curves of the above mentioned cultures. In addition, we tested the method on suspension cultures of onion and bacteria.